While excavating a cemetery full of medieval knights in Spain, archaeologists discovered the remains of a middle-aged man with two stab wounds on his head and a bashed-in knee, suggesting he died in battle. But when they took a closer look at the skeleton, they were shocked by his unusually long and narrow head, which they suspect resulted from a genetic condition that was typically fatal in childhood.

"I was very surprised," Carme Rissech, a biological anthropologist at the University of Rovira i Virgili in Spain, told Live Science in an email. "I had never seen a skull like this before, especially not one belonging to a knight."

In a study published Oct. 3 in the journal Heritage, Rissech and colleagues detailed their analysis of the bones of the skeleton, which was found at Zorita de los Canes castle in central Spain. The castle was occupied from the 13th to the 15th centuries by the Order of Calatrava, a group of knights and monks who took on military responsibilities.

When archaeologists excavated the Zorita de los Canes cemetery between 2014 and 2019, they uncovered dozens of human skeletons, including one of a woman, with traumatic injuries suggestive of violent incidents and battle wounds. But one individual stood out among the battered skeletons due to his "extremely elongated skull," the researchers wrote in the study.

The man had been buried in a wooden coffin that had largely disintegrated by the time it was excavated, and many of his bones had also decomposed over the centuries. A close study of his skeleton revealed he was in his mid- to late 40s when he died, and the muscle markers on his bones showed that he was an active person. But the researchers noticed that three of his cranial sutures — joints between skull bones — had closed prematurely, causing his head to be malformed.

When babies are born, their skull bones are basically small plates connected by fibrous joints called sutures. This flexibility lets the baby pass through the birth canal and allows the baby's brain room to grow. Most cranial sutures don't fuse together until a person is in their early 20s. If one or more sutures fuse too early — a condition called craniosynostosis — this can present problems for skull and brain growth. Today, surgery can be done to alleviate pressure on the brain caused by craniosynostosis, which can result in brain injury and death, but this sort of medical intervention was not available in medieval times.

The worldwide prevalence of craniosynostosis is about 1 in 2,500, according to the researchers, and many cases are the result of genetic mutations. One of the most common genetic mutations that causes multiple cranial sutures to fuse prematurely results in Crouzon syndrome, which can also cause wide-set, bulging eyes; a small jaw; and hearing loss. However, most people with this syndrome have normal cognitive function.

Because only the medieval knight's skull was affected and the rest of his skeleton was not, the researchers think he may have had Crouzon syndrome — a rare find among archaeological skeletons.

"Most documented cases — particularly in the medieval period — are pediatric," the researchers wrote. "The survival of this individual into adulthood without surgical intervention is especially noteworthy, given the potential complications."

However, the researchers cautioned that further genetic analysis is needed to prove that the man had Crouzon syndrome.

Still, this medieval man clearly survived and thrived in spite of a potentially life-threatening genetic condition. His bones "exhibit signs of an active lifestyle, which could be consistent with that of a warrior," the researchers wrote, and the stab wounds to his head "suggest that he could have died in battle."

Disease name: Harlequin ichthyosis, also called ichthyosis fetalis and harlequin baby syndrome

Affected populations: This genetic condition affects an estimated 1 in 300,000 live births globally. In the United States, it's estimated to occur in about 1 in 500,000 births, or about seven births a year, according to the National Organization for Rare Disorders. The condition seems to affect males and females in equal numbers and doesn't occur in greater frequency in any specific racial or ethnic group.

Causes: Harlequin ichthyosis is caused by a variety of mutations in a gene called ABCA12, which carries instructions for proteins that ship molecules across cell membranes. These proteins are key for transporting fats and enzymes in the outermost layer of the skin, known as the epidermis.

Most people with harlequin ichthyosis carry a genetic mutation that causes cells to make ABCA12 proteins that are too short and thus unable to properly transport fats. In some cases, people can't make the protein at all, which results in the most severe cases of the disease.

Evidence suggests that the condition is inherited in an autosomal recessive pattern, meaning a baby must inherit two copies of the broken gene — one from each parent — to develop the disorder. People with only one mutant copy of the gene are "carriers" but do not show symptoms. People with a family history of the disorder can consider seeking genetic counseling to see if they're a carrier.

Symptoms: The transportation of fat molecules in the epidermis is important for keeping the skin hydrated, and it's also necessary for the skin to develop properly in the womb and after birth. When babies have ABCA12 proteins that are too short or totally absent, these infants are born with thick, platelike scales of skin, which cause the skin to be stretched very tightly.

The tight skin cracks easily, forming fissures. The tension pulls at the affected baby's eyelids and lips, turning them inside out, and it also constricts the movement of the chest, which impedes breathing and eating. Poor nail and hair growth are also common.

Babies with harlequin ichthyosis are often born prematurely, and they may also show additional symptoms and traits, such as a flat nose, abnormal hearing, ears that are fused to their head, swollen hands and feet, and decreased joint mobility.

The condition compromises the skin's protective barrier, leaving the infant vulnerable to dehydration and unable to regulate their temperature or fight infections well. For infants that survive the newborn period, the "armor-like" plates eventually shed and the remaining skin is very red, dry and scaly.

Infants with harlequin ichthyosis often don't survive the newborn period, but with intensive medical care, it's possible for them to survive to later infancy, childhood and even early adulthood, in some cases. A report that reviewed 45 cases of the disease found that about 55% of the patients survived the newborn period, with survivors ranging from 10 months to 25 years old at the time of survey. Common causes of death in the period shortly after birth included respiratory failure and/or sepsis, a dangerous body-wide immune response.

Treatments: There is no cure for harlequin ichthyosis, so care aims to manage the symptoms of the disease.

According to the Cleveland Clinic, babies with the condition are cared for in the neonatal intensive care unit (NICU) immediately after birth. There, they can be placed in high-humidity incubators to help regulate their temperature and skin hydration. They're also bathed often to soften and loosen the skin plaques, and exfoliating techniques might be used to help remove the scales. Moisturizers and skin barrier repair formulas can also help with the excessive skin dryness and stiffness. Various pain relievers can help make newborns more comfortable.

In severe cases, medications called retinoids may be used and are typically given orally. These drugs help to break down the thick, platelike scales covering the skin, which, in turn, may reduce other symptoms of the condition. However, these medications are conserved for severe cases because they can have toxic side effects if used over the long term.

Antibiotics may be needed to prevent or counter skin infections. The eyes should also be lubricated regularly, as infants with the condition cannot close their eyelids properly.

Once discharged from the NICU, children with harlequin ichthyosis require ongoing skin care, as well as lifelong medical care to manage additional symptoms that may emerge due to their condition. This care usually requires a diverse team of medical specialists, including ophthalmologists, plastic surgeons, nutritionists, physical and occupational therapists, and speech and language therapists.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Werner syndrome, sometimes called "adult progeria"

Affected populations: Werner syndrome is estimated to affect 1 in 100,000 live births worldwide, though its prevalence varies among countries. In Japan, the syndrome affects an estimated 1 in 40,000 to 1 in 20,000 people, whereas the prevalence in the United States is around 1 in 200,000. This difference between locations is partly attributed to "founder effects" — instances where genetic variation declines after a small group of individuals gets separated from a larger population. This limits the gene pool and can cause disease-causing mutations to become more widespread within a population.

Werner syndrome affects males and females at equal rates.

Causes: Werner syndrome is caused by mutations in the WRN gene, which is needed to make the so-called Werner protein. This crucial enzyme unwinds and separates the two strands of a DNA molecule and also removes bits of damaged DNA. This helps cells to repair their DNA following damage, make copies of their DNA as they multiply and use the genetic instructions to make proteins.

The enzyme may also help maintain telomeres, the protective "caps" at the ends of DNA molecules that prevent them from unraveling like frayed shoelaces. Telomeres get shorter with age, and the rate at which they shorten is tied to the rate of biological aging observed and expected lifespan of an organism.

More than 80 different mutations in the WRN gene have been found in people with Werner syndrome. Most often, such mutations cause cells to make a version of the Werner protein that's too short and doesn't work. The exact consequences of this aren't fully understood, but laboratory studies suggest that cells carrying these mutations can't divide as many times as cells without the mutations can. They also enter senescence — a zombie-like state associated with cellular aging — earlier than healthy cells do. Additionally, mutations in WRN may prevent cells from correcting DNA damage, allowing other, harmful mutations to accumulate.

The syndrome is inherited in an autosomal recessive pattern, meaning it's caused by a person inheriting two broken copies of the WRN gene — one from each parent.

Symptoms: The symptoms of Werner syndrome typically start to emerge in the second decade of life, around adolescence. Children with Werner syndrome are often thin and have a slow growth rate later in childhood, sometimes missing the usual growth spurt seen in adolescence.

People with the syndrome may start growing gray hair before age 20. By 25, they start to lose hair from the scalp, eyebrows and eyelashes, and they may grow only sparse hair elsewhere on the body, including the underarms and chest. This lack of hair is likely related to hypogonadism, in which the ovaries or testes don't work well; hypogonadism also undermines the development of sexual organs and the regularity of menstruation.

Werner syndrome also causes people to lose the layer of fat beneath the skin, along with muscle mass and bone density. Atrophy of the vocal cords causes many people to develop a high-pitched, squeaky or hoarse voice. Their skin develops smooth or hard patches, areas of hyper- or hypopigmentation, and redness due to widened blood vessels. Together, these changes cause people with the syndrome to have a "pinched"-looking face, with prominent eyes and a thin, beaked nose.

In some people, soft tissues such as ligaments and tendons calcify over time, becoming stiffer. Calcium can also build up in the cornea of the eyes. By their 20s or 30s, many people with Werner syndrome develop age-related cataracts, a clouding of the lenses of the eyes that usually does not occur before age 50. In their 30s, people with Werner syndrome can develop type 2 diabetes, hardening of the arteries (atherosclerosis), chest pain (angina), heart attack or heart failure.

People with the syndrome are also prone to certain cancers, such as thyroid cancer, melanoma, osteosarcoma and soft tissue sarcoma. Studies conducted in the 2000s suggested people with the syndrome typically die in their early to mid-50s, but with newer medical treatments, people can now survive a few years longer.

Treatments: There is no cure for the underlying cause of Werner syndrome; treatments are aimed at addressing a patient's specific symptoms. For example, a person may take medications and implement dietary and lifestyle changes to manage type 2 diabetes; undergo surgery and chemotherapy for cancer; and take medicines to counter the hardening of their arteries.

Diagnosing Werner syndrome may involve genetic testing to confirm that a person carries two defective copies of the WRN gene. In addition, the family members of people with Werner syndrome can undergo genetic counseling to see if they're carriers of mutant WRN genes; those who carry only one copy don't develop the syndrome but could pass on the condition to their children if their partner also carries a mutant copy.

Couples who are carriers but still wish to conceive a child can potentially explore preimplantation genetic testing, which is genetic testing performed as part of an in vitro fertilization (IVF) procedure.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Moebius syndrome

Affected populations: The exact incidence of Moebius syndrome is unknown. A 2021 survey in Italy suggested that the condition affects 0.3 in 100,000 newborns in the country, while a 1996 Dutch study estimated that it affects about 2.1 per 100,000 newborns in The Netherlands. Other studies estimate that, worldwide, between 1 in 500,000 and 1 in 50,000 babies are born with the syndrome.

These estimates may vary because the diagnostic criteria for Moebius syndrome have changed over time. Plus, due to the condition's rarity, countries don't maintain national registries to track its incidence and physicians often lack knowledge of the condition, complicating diagnosis, the authors of the Italian study suggested. The syndrome is thought to occur in people of all races and ethnicities and to affect males and females at equal rates.

Causes: The key feature of Moebius syndrome is the absence or underdevelopment of two sets of cranial nerves (CN): the abducens nerves (CN VI) and facial nerves (CN VII).

There are 12 pairs of cranial nerves in total, and they relay signals from the brain to different parts of the face, head, neck and torso. The abducens nerves control the muscles that move the eyes away from the centerline of the face, moving the left eye left and right eye right. The facial nerves control various movements in the face, such as blinking, scrunching the forehead, smiling and frowning; they also relay sensory signals related to hearing and taste and help control the saliva and tear glands. Depending on the patient, Moebius syndrome may be associated with problems in other cranial nerves, in addition to CN VI and VII.

The exact causes of these cranial nerve problems in Moebius syndrome are not well understood. Most cases are sporadic, meaning they occur randomly rather than running in families. However, in a small percentage of cases, the syndrome appears to run in families, but the pattern of inheritance isn't consistent.

Some studies have pinpointed mutations in certain genes that may contribute to at least some cases of the disorder. These genes include PLXND1 and REV3L, which respectively contribute to brain development in the womb and DNA repair.

The risk of Moebius syndrome has also been linked to exposure to certain drugs in the womb. These substances include alcohol, cocaine, benzodiazepines, the immunosuppressant thalidomide and the migraine medication ergotamine. Many researchers think that the syndrome may emerge due to disruptions in blood flow to the brainstem during early stages of development.

Symptoms: The symptoms of Moebius syndrome can vary from person to person. That said, to be diagnosed patients generally have the following characteristics: facial paralysis or weakness affecting at least one side of the face and an inability to do sideways eye movements while still being able to look up and down.

Infants with the syndrome may drool more than is typical, have crossed eyes and need to move their whole head to track objects along a horizontal path. They typically show no facial expressions, which is very apparent when they're laughing or crying. They also often have trouble feeding as their suckling and swallowing abilities are affected.

Additional symptoms seen in some people with Moebius syndrome are having a small jaw and short tongue. Some have cleft palates, abnormalities in the structure of the external ear, or hearing loss, if CN VIII is affected. Some have limb differences, such as clubbed feet or webbed fingers. Many experience delays in developing motor skills, such as crawling, but then catch up later in development.

Historically, research tied the syndrome to an increased risk of autism spectrum disorder, but that link has been questioned in recent years. It's thought that patients may have been misdiagnosed as autistic due to differences stemming from nerve palsy — such as speech delay and an inability to make facial expressions — that make social interactions more difficult.

With treatment, people with Moebius syndrome typically have normal lifespans.

Treatments: Treatments and therapies for Moebius syndrome are tailored around each patient's set of symptoms.

To restore movement to the face, doctors may recommend procedures that transfer muscles and/or nerves from another part of the body into the affected parts of the face. These include surgeries that can help improve the individual's ability to chew, blink, smile and execute other facial movements and expressions.

In infancy, children with the syndrome may need feeding tubes to get adequate nutrition. Because patients have trouble blinking, it's important to keep the eyes lubricated with products like eye drops and to protect the eyes from sun using sunglasses and widebrimmed hats. Patients with crossed eyes can also undergo surgery to realign the eyeballs, if their alignment doesn't improve on its own as they grow.

Physical and occupational therapy can be helpful for patients with orthopedic abnormalities, as can speech therapy for patients with speech delay. Surgeries or physical supports, such as splints, braces and prostheses, may aid patients with limb differences.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Prader-Willi syndrome

Affected populations: This genetic, multisystem disorder affects an estimated 1 in 30,000 to 1 in 10,000 people worldwide. Most cases of the syndrome occur sporadically, meaning the genetic changes behind the condition appear randomly in early development rather than being passed down through generations. Only in rare cases is the genetic trigger inherited.

The syndrome affects males and females at equal rates, as well as people of different racial and ethnic backgrounds. Approximately 10,000 to 20,000 people in the United States have Prader-Willi syndrome.

Causes: The syndrome occurs when specific genes on chromosome 15 lose their function, either because they are missing or because they've been turned off. The portion of chromosome 15 that's affected in Prader-Willi syndrome is called 15q11.2-q13, and it's also known as the Prader-Willi syndrome/Angelman syndrome (PWS/AS) region.

Human cells typically carry 46 chromosomes, including 22 pairs of non-sex chromosomes, numbered 1 through 22, and one pair of sex chromosomes. Each parent contributes one-half of each pair of non-sex chromosomes — so one copy of chromosome 15 comes from the mother and another comes from the father, for example. Because cells carry two copies, every gene on each chromosome does not need to be active. Through a process called "genetic imprinting," certain genes on either the mother's or the father's copy get switched off.

Prader-Willi syndrome affects the paternal copy of chromosome 15, and in most cases — around 60% or 70% — the PWS/AS region is randomly deleted during development. Meanwhile, the maternal PWS/AS region is switched off in everyone. Thus, the deletion leaves people without a working set of those genes.

For about 30% to 40% of people with the syndrome, they inherit two copies of the maternal chromosome 15, so they're missing the paternal copy altogether.

More rarely, a person carries the paternal chromosome 15 but the relevant genes do not work properly. This happens due to either a small genetic mutation — a "microdeletion" — or changes in epigenetics, which are modifications to DNA that don't change its code but can turn a gene on or off. Even more rarely, the syndrome may be triggered by translocation, in which a portion of chromosome 15 breaks off and then reattaches to a different chromosome.

Symptoms: Prader-Willi syndrome affects many parts of the body, and its symptoms can vary from person to person. Many of the genetic changes that underlie the syndrome are thought to affect the hypothalamus, a key hormone-making region of the brain that helps control many basic bodily functions, including body temperature, hunger and sleep.

Hypotonia, or low muscle tone, is seen in nearly all infants with the syndrome, making them feel "floppy" when held. There can be signs of this symptom before birth, in that the fetus will not move as much as expected or will end up in unusual positions. After birth, hypotonia can contribute to the baby having a poor sucking reflex, which hinders feeding and weight gain in early life. Developmental delays are also common.

Infants can also have distinctive features, such as almond-shaped eyes; a thin upper lip; a downturned mouth; and a long, narrow head. Many patients have short stature, in part due to growth hormone deficiency. A percentage of people with the syndrome lose a gene called OCA2, which is involved in making pigment in the skin and hair. These individuals have very fair skin and light-colored hair.

Later in childhood, around age 2 to 8, children's appetite often increases dramatically, and these individuals tend to not feel full after meals. This can subsequently lead to hyperphagia, or excessive eating, which can result in obesity and related complications, such as type 2 diabetes, heart issues and gastrointestinal problems. It's thought that this voracious drive to eat is triggered by issues with the hormones that normally regulate appetite, as well as by differences in brain areas involved with processing rewarding stimuli (like food).

Additional symptoms of Prader-Willi syndrome include cognitive impairment, ranging from mild to moderate; genital underdevelopment; sleep problems; nearsightedness; and underactive thyroid.

With adequate treatment and support, people with Prader-Willi syndrome can live into their 70s. But complications such as diabetes and heart failure can limit their lifespan if these conditions are not adequately controlled, and they can cause death by a person's 40s.

Treatments: There is no cure for Prader-Willi syndrome. Treatments vary depending which symptoms a person has, when those symptoms began and how severe they are.

To help patients feed in infancy, doctors may recommend high-calorie formula and special feeding methods, including tube feeding. Replacing missing hormones — with testosterone, estrogen or growth hormone, for example — can help offset symptoms related to low hormone levels. Growth hormone therapy was approved as a Prader-Willi syndrome treatment by the Food and Drug Administration (FDA) in 2000, and it's been shown to help boost muscle tone and growth while lowering body fat.

Physical, behavioral, occupational and speech therapies can help with motor skills, intellectual disabilities, and speech and language development. Medications might be recommended to help with sleep issues or psychiatric disorders that can be associated with the syndrome, such as psychosis.

The FDA also approved the first-ever treatment for hyperphagia in Prader-Willi syndrome in 2025. It's approved for patients ages 4 and older. The exact mechanism of this drug is unknown, but it's thought to help decrease the production of certain hunger-triggering signals from the hypothalamus.

Especially in childhood, patients with Prader-Willi syndrome should be kept on carefully controlled diets and eating schedules, the Mayo Clinic advises. Dietitians can help advise patients and their families on what to include in a healthy diet and whether supplemental vitamins or minerals might be necessary.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Xeroderma pigmentosum (XP)

Affected populations: Xeroderma pigmentosum is a rare genetic disorder that makes people's skin extremely sensitive to the harmful effects of ultraviolet (UV) light. Both males and females can develop XP, and it can affect people from different ethnic backgrounds.

XP is estimated to affect approximately 1 person per million in the United States, but it is more common in other regions of the world, including Japan, North Africa and the Middle East. For example, in Japan, around 45 people per million are estimated to have XP.

Related: Simple blood test could reveal likelihood of deadly skin cancer returning, study suggests

Causes: XP is caused by gene mutations that prevent DNA from being repaired after it's damaged by UV radiation, which is present in sunlight and some kinds of artificial lighting, including that used for tanning beds. The unrepaired DNA accumulates within cells, triggering a plethora of different symptoms.

Scientists have identified eight different gene mutations that are tied to the development of XP; the most common of these, at least in the U.S., is a mutation in a gene called XPC. This gene codes for an enzyme that is key for detecting DNA damage, and the mutation renders the protein unable to do its job.

XP is an autosomal recessive condition, which means that people must inherit two copies of a faulty gene — one from each parent — in order to develop it.

Symptoms: Symptoms of XP usually begin in infancy or early childhood. People with the condition typically burn very easily in the sun — around half of children with XP are susceptible to severe sunburn within only a few minutes of exposure to sunlight.

Patients with XP may also develop freckles before the age of 2 in parts of the body that are commonly exposed to daylight, such as the face, arms and lips. In addition, their eyes are often very sensitive to DNA damage caused by bright light. Repeated exposure to sunlight may additionally cause the skin to become dry, thinner and have a mixture of darker and lighter regions.

Patients with XP are up to 2,000 times more likely to develop melanoma, a type of skin cancer for which UV exposure is a major risk factor. Without protection from UV light, people with XP are likely to develop skin cancer many times in their life — most commonly for the first time before age 10.

Around 25% of people with XP will additionally develop neurological symptoms associated with the disorder, likely because of damage to DNA in nerve cells in the brain. These symptoms progressively develop over a patient's lifetime, regardless of their level of sun exposure, and include hearing loss, poor coordination and balance, and intellectual disability.

People with XP, on average, live to around 37 years old. However, those who also have neurological symptoms of the condition usually live to age 29.

Treatments: There is currently no cure for XP. However, patients are advised to always be completely protected from sunlight. Being consistently shielded against UV radiation — by using sunscreen and wearing layered clothing and sunglasses — can considerably reduce the number of skin cancers that patients develop.

Other treatments include surgery to remove skin cancer if it develops; hearing aids to help with hearing loss; and eye drops to help lubricate sore eyes. Patients may also be advised to take vitamin D supplements to top up their levels of this vitamin, which may otherwise be too low due to a lack of sun exposure.

Those with the neurological symptoms of XP may additionally eventually need to use a wheelchair, be tube fed and receive long-term nursing care.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: La Crosse virus disease

Affected populations: La Crosse virus disease is a rare viral illness that is spread to humans via mosquito bites. The name comes from La Crosse County, Wisconsin, where the disease was first observed by doctors in the 1960s.

Approximately 30 to 90 cases of La Crosse virus disease are reported in the U.S. each year. Around 60% of those cases occur in males, and 90% affect people under 20 years old.

Transmission of La Crosse virus most often happens in late spring through early fall, when mosquito populations peak. Most cases of the disease occur in upper Midwestern, Mid-Atlantic and Southeastern U.S. states. No cases have ever been reported outside the U.S.

Related: New York resident dies of eastern equine encephalitis — what is it?

Causes: La Crosse virus disease is caused by the La Crosse virus, which is spread to humans via the bites of mosquitoes carrying the germ. Most commonly, a species known as Aedes triseriatus spreads the virus to humans. As their name suggests, these mosquitoes typically breed in holes within trees, and they also lay eggs in outdoor containers that hold standing water.

People who live, work or spend recreational time in wooded areas have a higher risk of becoming infected with La Crosse virus than others, as they are more likely to come into contact with A. triseriatus mosquitoes.

Once inside the human body, the La Crosse virus can infiltrate the central nervous system, including the brain, and infect and damage neurons there.

La Crosse virus disease cannot be spread from person to person; the only way people can catch it is directly from mosquito bites. Mosquitoes can't pick up the virus from infected people, because it never reaches a high enough concentration in the blood. For this reason, humans are considered "dead-end" hosts of the La Crosse virus. (In the environment, mosquitoes pick up the virus from small mammals, such as squirrels and chipmunks.)

Symptoms: Around 96% of people exposed to La Crosse virus do not develop any symptoms. The small percentage who do may initially experience a fever, headache, nausea and vomiting. These symptoms can emerge within three to 10 days of being bitten by an infected mosquito.

For some people, especially children under 16, these symptoms can then progress to more severe disease. For instance, the virus may cause brain inflammation, known as encephalitis. Around 75% of the La Crosse cases that involve encephalitis occur in children.

The fatality rate from La Crosse encephalitis is less than 1%. However, between 5% and 15% of patients who recover from this condition can go on to experience recurring seizures after the initial infection has passed.

Treatments: There is no cure for La Crosse virus disease, nor a vaccine that can help prevent infection. However, patients with severe symptoms may be given supportive care in the hospital.

People can reduce their risk of being infected with La Crosse virus by avoiding mosquito bites in the first place. They can do this by using recommended insect repellents and wearing long-sleeved shirts and pants while outdoors, the Centers for Disease Control and Prevention recommends.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Lesch-Nyhan syndrome

Affected populations: Lesch-Nyhan syndrome is an incredibly rare genetic disorder that causes metabolic changes and mainly affects males. Approximately 1 in 380,000 babies born in the United States are estimated to have the condition.

Causes: Lesch-Nyhan syndrome is caused by mutations in a gene called HPRT1. This gene carries instructions for an enzyme that cells use to recycle important chemical compounds called purines; these are found, for instance, in DNA and in molecules that interact with enzymes. Purines are also present in many foods, such as offal (organ meats), poultry and legumes.

In patients with Lesch-Nyhan syndrome, the HPRT1 enzyme doesn't work properly due to mutations in its associated gene. Therefore, cells cannot recycle purines as they normally would, and a waste product called uric acid consequently builds up in people's blood.

Related: New mRNA therapy shows promise in treating 'ultrarare' inherited disease

This excess uric acid also clumps into small stones or crystals in different regions of the body, including in the joints and kidneys.

For unknown reasons, patients with Lesch-Nyhan syndrome also have lower levels of the chemical messenger dopamine in their brains, compared with people without the condition. Dopamine plays a key role in controlling movement and processing emotions, among other abilities.

The HPRT1 gene is found on the X chromosome, one of the two sex chromosomes in humans; females typically have two X chromosomes (XX), while males usually have an X and a Y chromosome (XY).

Because females have two X chromosomes, if the HPRT1 gene on one of those chromosomes is mutated, the non-mutated version on their other chromosome can effectively compensate for it. However, because males have only one X chromosome, if they have a mutant HPRT1 gene, they will develop Lesch-Nyhan syndrome. That is why the condition almost exclusively affects males.

Although the syndrome tends to run in families, it can sometimes occur in individuals with no family history of the condition, due to random mutations that emerge in HPRT1 during pregnancy.

So far, more than 600 mutant versions of the HPRT1 gene have been found to be associated with Lesch-Nyhan syndrome.

Symptoms: The main symptoms of Lesch-Nyhan syndrome are associated with inflammatory arthritis — a group of conditions driven by the immune system that cause joint pain, swelling and tenderness. Specifically, people with the syndrome often have gout.

People may also experience kidney and bladder stones and cognitive disability. They may have involuntary muscle movements and engage in self-injurious behaviors, such as lip and finger biting and head banging. These uncontrollable behaviors may be driven by the loss of, or damage to, dopamine neurons in the brain.

Often, patients need help walking and sitting and many require a wheelchair.

Symptoms of Lesch-Nyhan syndrome typically emerge before a child reaches their first birthday.

The main causes of death in patients with Lesch-Nyhan syndrome are a bacterial infection of the lungs called aspiration pneumonia and kidney failure. The aspiration pneumonia often happens because patients develop severe swallowing issues.

Treatments: There is no cure for Lesch-Nyhan syndrome, but with effective clinical care, patients can live until around age 40.

Treatments that can help manage patients' symptoms include muscle relaxants to control involuntary movements, as well as physical restraints or teeth extraction to stop them from injuring themselves.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Hantavirus disease

Affected populations: Hantavirus disease is an uncommon but potentially deadly infection caused by a family of viruses called hantaviruses. These viruses are found worldwide and are typically carried and spread by rodents, such as rats and mice. Different types of hantaviruses are associated with specific rodent species, which can carry the viruses without overt signs of illness.

Most hantaviruses cannot spread from person to person, but one type — the Andes virus, found in South America — can do so.

The United States began tracking hantavirus disease in 1993, and between then and 2023, 890 cases were reported within the country. The states with the highest number of reported cases include Washington, California, Arizona, Colorado and New Mexico. An imported case of Andes virus was reported in 2018 in Delaware after a traveler had been exposed in South America.

"Worldwide, it is estimated that from 10,000 to over 100,000 infections occur each year, with the largest burden in Asia and Europe," the World Health Organization (WHO) says. Historically, the Americas have reported hundreds of cases each year.

Given that rodents spread hantaviruses, people who are more likely to encounter these animals have a greater chance of developing hantavirus disease. So forestry workers, farmers and trappers are at higher risk than the general population.

Causes: Humans can develop hantavirus disease after being exposed to the urine, droppings or saliva of infected rodents. This can happen if a person rubs their eyes after touching bodily fluids or poop carrying the virus. Additionally, if someone disrupts debris containing infected animal droppings — while cleaning a barn, for instance — hantaviruses can be released into the air and then inhaled. On rare occasions, people may develop hantavirus disease after being bitten by an infected rodent.

The Andes virus, which can transmit between people, has been associated with several clusters of infection in recent years, including the 2026 cluster associated with the cruise ship MV Hondius.

Once inside the body, hantaviruses can cause two types of serious infections: hantavirus cardiopulmonary syndrome (HCPS) and hemorrhagic fever with renal syndrome (HFRS). (Note that HCPS is sometimes called hantavirus pulmonary syndrome; these two terms refer to the same condition.)

Hantaviruses in the Americas are known to cause HCPS, while those in Europe and Asia cause HFRS.

Symptoms: Early symptoms of HCPS include fever, muscle aches and fatigue, which can take one to eight weeks to develop following exposure to a hantavirus. Additional symptoms include chills, headaches, dizziness and gastrointestinal issues. Within four to 10 days of the first symptoms, the disease can progress to cause coughing, shortness of breath, shock and fluid buildup in the lungs.

Approximately 38% of patients who develop these respiratory symptoms die from the disease, the U.S. Centers for Disease Control and Prevention (CDC) states. Case fatality rates up to 50% have been reported in some contexts, the WHO says.

By contrast, HFRS is less deadly, with case fatality rates between 1% and 15% depending on the virus at fault. Symptoms usually emerge within one to two weeks after exposure to a hantavirus, but they can take up to eight weeks to appear. Early symptoms include intense headaches, back and abdominal pain, fever, chills, nausea and blurred vision. Later stages of the disease involve low blood pressure, lack of blood flow, internal bleeding and kidney failure.

Treatments: There is no cure for hantavirus disease. Instead, treatment aims to manage a patient's symptoms and stabilize their vitals. Doctors may provide a patient breathing support if they have respiratory issues, or dialysis if their kidneys are too damaged to filter blood properly.

Prevention: To reduce exposure to hantaviruses in the first place, the U.S. Centers for Disease Control and Prevention (CDC) recommends that people eliminate or minimize their contact with wild rodents. They can do this by storing food securely, sealing any holes or gaps in their houses or garages to prevent rodents from entering, and using traps when rodents have already infiltrated, for example.

Tips for safely cleaning up rodent droppings can be found on the CDC website.

When it comes to the Andes virus, which can spread between people, healthcare providers caring for infected patients should wear personal protective equipment and employ other standard safety procedures, such as hand hygiene, environmental cleaning, and safe handling of blood and bodily fluids.

"During outbreaks or when cases are suspected, early identification and isolation of cases, monitoring of close contacts, and application of standard infection prevention measures are important to limit further spread," the WHO says.

For people outside healthcare, the precautions recommended to avoid Andes virus infection are similar to those recommended to avoid more common illnesses like the flu or common cold. These include avoiding kissing or sexual contact with potentially infected people, as well as avoiding sharing drinks and eating utensils. It's also advisable to wash hands frequently and maintain distance from potentially infected people.

Recent cases: In February 2025, the American classical pianist and businesswoman Betsy Arakawa, who was also the wife of actor Gene Hackman, reportedly died from HCPS.

In April 2026, the cruise ship MV Hondius was struck by a cluster of hantavirus cases that were later confirmed to be caused by the Andes virus, the only hantavirus known to spread from person to person.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Nocardiosis

Affected populations: Nocardiosis is a rare but potentially deadly infectious disease caused by bacteria in the genus Nocardia. Nocardiosis is an opportunistic infection, meaning it doesn't typically affect healthy people but may seize the chance to infect people with weakened immune systems, such as people with cancer or HIV/AIDS, as well as organ transplant recipients who are taking immunosuppressive drugs.

However, around 20% to 30% of patients with nocardiosis have no known pre-existing conditions, so the infection doesn't exclusively affect people with immune deficits. People over the age of 40, especially men, are also more likely to develop the disease than other demographics.

Between 500 and 1,000 new cases of nocardiosis are reported in the United States every year.

Related: Scientists have found a secret 'switch' that lets bacteria resist antibiotics — and it's been evading lab tests for decades

Causes: Nocardia bacteria are found in soil, standing water and decaying plant material. Around 100 Nocardia species have been identified so far, of which 12 are known to infect humans.

People may become infected with Nocardia bacteria when they inhale dust containing the microbes or they have a cut or scrape that comes in contact with contaminated soil or water.

Nocardiosis is not known to spread from one person to another; people pick it up directly from the environment.

Symptoms: The symptoms of nocardiosis vary depending on which part of the body is infected by Nocardia bacteria.

Most cases of nocardiosis start out as lung infections, in which pus-filled cavities, or abscesses containing the bacteria, form in the lungs. This can cause symptoms such as chest pain, a cough (including coughing up blood), sweats, chills and general weakness.

Nocardia bacteria can then travel in the bloodstream and form abscesses in other regions of the body, including the brain, kidneys and intestines. Infections of the brain can cause headache, weakness, confusion and seizures.

Approximately one-third of all patients infected with nocardiosis develop skin ulcers or sores, instead of an internal infection. These skin lesions typically form across the hands, chest wall or buttocks. They may look like open wounds or bumps under the skin.

Without treatment, nocardiosis can rapidly lead to death, often by causing organ failure or sepsis, a dangerous body-wide immune reaction. Between 16% and 40% of patients with nocardiosis die as a result of their infection. If the disease spreads to the brain, death rates jump to more than 80%.

Treatments: Nocardiosis can be treated with common antibiotics, although these bacteria are normally resistant to penicillin. The antibiotic treatment usually takes between six and 12 months to complete, and some patients may need to take antibiotics for even longer to prevent the disease from coming back.

Surgery may sometimes also be required to remove specific abscesses from the body, especially if a patient is not responding to antibiotic treatment.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Trimethylaminuria (TMAU), also known as "fish odor syndrome"

Affected populations: TMAU is a rare metabolic condition that causes a person to smell like rotten fish. The condition is more common in women than in men, and there's evidence that female sex hormones, such as progesterone, can exacerbate patients' symptoms.

The exact prevalence of TMAU is unknown, and estimates of global cases vary greatly, ranging from 1 in a million to 1 in 200,000 people.

Causes: Patients with TMAU smell like fish due to a buildup of a chemical called trimethylamine in their body. Trimethylamine is produced by bacteria in the gut as a byproduct of the digestion of certain foods, including eggs, liver, legumes and specific kinds of seafood, such as fish, squid and crabs.

Related: Why can't we smell ourselves as well as we smell others?

Normally, an enzyme in the body breaks down trimethylamine into an odorless chemical, known as trimethylamine N-oxide, which is then excreted via urine. This enzyme is encoded by a gene called FMO3.

In patients with TMAU, though, this enzymatic process doesn't occur, so trimethylamine accumulates in the body and ends up being released in excess quantities in patients' sweat, urine and breath. This makes them smell like rotten fish.

Most cases of TMAU are caused by mutations in the FMO3 gene that prevent the enzyme it encodes from working properly. In these instances, patients inherit the disease in an autosomal recessive manner, meaning they must inherit two copies of the mutated FMO3 gene — one from each parent — to develop the condition.

More rarely, TMAU can be caused by consuming a large quantity of foods that lead to trimethylamine production. It can also result from liver failure and certain medical treatments, such as testosterone replacement therapy, which impact the processing and production of trimethylamine, respectively. Hormonal changes brought about by the menstrual cycle can also cause a transient form of TMAU.

Symptoms: Symptoms of TMAU may be present from birth or arise later in life, normally near the start of puberty (roughly around age 8 to 13 in females and 9 to 14 in males), when many hormonal changes happen.

Some patients with TMAU have a strong fishy odor all the time, while the smell may come and go for others with the condition. A patient's stress levels and diet can worsen their symptoms by increasing their sweat production and levels of trimethylamine, respectively.

TMAU is not deadly, but the condition can have devastating effects on patients' quality of life, by impeding their relationships with others and their career, for instance. These impacts can considerably impede their mental health and may lead to symptoms of depression, anxiety and suicidal thoughts in some.

Treatments: There is no cure for TMAU. However, health care providers may recommend that patients avoid foods that contain trimethylamine or substances that can be broken down into the chemical. These include milk from wheat-fed cows, as well as eggs, liver, kidney, seafood and peas.

Patients may also be advised to wash their skin with a slightly acidic soap or shampoo, to avoid strenuous exercise that causes sweating, to wash their clothes frequently and to use antiperspirant. They may also be advised to take measures to reduce their stress levels.

Additionally, doctors can prescribe low doses of antibiotics to reduce the amount of bacteria in the gut that metabolize trimethylamine. They may also prescribe activated charcoal, which binds to and reduces the amount of trimethylamine that can be absorbed from the gut. (Activated charcoal can interact with many medications, though, so it should be used with caution.)

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Babesiosis

Affected populations: Babesiosis is a rare and potentially fatal parasitic disease that destroys red blood cells, the cells that supply tissues with oxygen from the lungs. The disease, which is spread by ticks, occurs worldwide, including in the United States and Europe.

Fewer than 3,000 cases of babesiosis are reported annually in the U.S., and they most commonly occur between May and September in the upper Midwest and Northeast, including in Minnesota, Wisconsin, Connecticut and New York. Cases tend to rise in the spring and summer as this is when people are most likely to be in contact with the ticks that spread the disease.

Causes: Babesiosis is caused by microscopic parasites that belong to the genus Babesia. These parasites usually infect cattle and are spread between animals by ticks that eat the blood of different hosts.

Related: Tick-borne parasite is spreading in the Northeast, CDC says

Once inside the body, Babesia parasites invade and destroy red blood cells. This severely limits the ability of these cells to supply tissues with oxygen.

While more than 100 species of Babesia parasites have been identified, overall, only a few species are known to infect humans.

In the U.S., most babesiosis infections are caused by a parasite species called Babesia microti and are spread by blacklegged ticks (Ixodes scapularis), also known as deer ticks. These ticks are typically found in wooded, brushy or grassy areas.

In rarer instances, Babesia parasites can be spread from one person to another via contaminated blood transfusions, and they can also spread from mother to fetus across the placenta.

Symptoms: Most people exposed to Babesia parasites don't have any symptoms of babesiosis; this is especially true for young, healthy people.

However, in individuals who have weakened immune systems or who are over the age of 50, the parasites can trigger severe disease. People who have had their spleen removed are also more vulnerable to serious infections than the average person, because the spleen normally helps remove infected red blood cells from the body.

Typical symptoms of babesiosis include fever, chills, sweating, muscle aches and pains, as well as swelling of the liver and spleen and having a low level of red blood cells in the body. Symptoms usually emerge within one to four weeks of a person being infected with Babesia parasites, and they can last for several days after onset.

Serious cases of babesiosis can cause multiorgan failure and death, as tissues are starved of oxygen.

Estimates for death rates from babesiosis vary considerably between studies. However, surveillance data gathered in 2019 by the Centers for Disease Control and Prevention (CDC) found a 0.57% death rate among patients in the U.S.

Death rates may be closer to 20% in patients who belong to high-risk groups, even when they receive treatment.

Treatments: Patients who don't have symptoms of babesiosis usually don't require treatment, as the immune system will typically clear the parasites away within one to two weeks.

In symptomatic patients, the main treatment for babesiosis is a combination of antiparasitic drugs and antibiotics. The latter drugs are primarily used to treat bacterial infections, rather than parasitic infections, but certain kinds of antibiotics, such as clindamycin, can also be effective against parasites.

Patients who are very sick may also require a blood transfusion to replace their damaged and infected red blood cells.

The best way to prevent babesiosis is to avoid areas where ticks live, according to the CDC. If you are in those areas, there are precautions you can take to avoid tick bites.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Polymerase gamma-related diseases, called POLG-related diseases for short

Affected populations: POLG-related diseases are a group of rare, incurable conditions caused by inherited gene mutations. These disease-causing mutations are carried by up to 2% of people of Northern European descent. However, not everyone who carries the mutations ultimately develops a POLG-related disorder.

Estimates suggest that these diseases affect approximately 1 in 10,000 people worldwide.

Causes: POLG-related diseases are caused by several hundred different mutations in the POLG gene. This gene encodes a protein in mitochondria, called DNA polymerase gamma (pol γ), which is needed to replicate and repair the unique DNA found only inside mitochondria, rather than in the cell nucleus.

People develop POLG-related diseases by inheriting either one or two copies of the mutated POLG gene from one or both of their parents.

Mitochondria are the cellular powerhouses responsible for breaking down the carbohydrates and fatty acids in food to produce molecules called ATP, which the body then uses as energy.

Related: Malfunctioning mitochondria may drive Crohn's disease, early study hints

In people with POLG-related diseases, issues with polymerase gamma can either end up decreasing the amount of DNA in mitochondria or introducing mutations into that DNA. And in some cases, both these events can occur.

Ultimately, cells become energy-depleted, and this can lead to organ dysfunction and failure.

POLG mutations are the most common cause of inherited mitochondrial disorders, with around 300 mutations in POLG known to cause disease. Examples of POLG-related diseases include Alpers-Huttenlocher syndrome, ataxia neuropathy spectrum and childhood myocerebrohepatopathy spectrum.

Symptoms: POLG-related diseases can be tricky to diagnose, in part, because they can cause an array of symptoms that vary in severity. These symptoms can also emerge at different times in a person's life, from early childhood to adulthood, depending on which disease they have.

The timing of symptoms partly varies depending on the specific POLG mutation a person carries and the number of copies, as well as the underlying driver of their condition — whether it's mostly related to the loss of DNA or accumulation of mutations.

Once they manifest, the symptoms mostly affect parts of the body that require a lot of energy, such as the central nervous system, muscles and liver.

Common symptoms of POLG-related diseases include progressive weakness of the eye muscles and the muscles that are closest to the center of the body, such as those located in the shoulders or upper arms. People with these diseases may also have droopy eyelids and develop epilepsy and liver failure.

Alpers-Huttenlocher syndrome is one of the most severe forms of POLG-related disease. In addition to the previously listed symptoms, people with this condition also usually experience seizures, a reduced ability to move and liver disease.

People with ataxia neuropathy spectrum, meanwhile experience issues with coordination and balance, as well as reduced nerve function.

Treatments: There is no cure for POLG-related diseases; current treatments instead focus on reducing a patient's symptoms. For instance, doctors can prescribe drugs to help manage seizures or movement difficulties. Transplantation is also an option for patients who experience liver failure.

People with a POLG-related disease typically live for between three months and 12 years after symptoms first begin.

Recent cases: In March 2025, Prince Frederik of Luxembourg, who was second cousin of the heir to the throne, reportedly died of a POLG-related disease at the age of 22. Frederik was the founder and creative director of The POLG Foundation, an organization that supports research into treatments for POLG-related diseases.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Progressive familial intrahepatic cholestasis (PFIC)

Affected populations: PFIC is a group of rare genetic diseases that cause progressive liver failure. The exact prevalence of PFIC is unknown, but estimates suggest that these diseases affect between 1 in 100,000 and 1 in 50,000 people worldwide. In the United States, fewer than 50,000 people total are believed to have PFIC.

Causes: Patients with PFIC have genetic mutations that impair the liver's ability to secrete a digestive fluid known as bile into the digestive tract.

Bile is a yellowish-green fluid produced in the liver, and it is normally secreted into the digestive tract to help with the breakdown of fats, the absorption of vitamins from food, and the removal of waste products in stool.

But in patients with PFIC, bile instead accumulates in the liver and thus begins to damage the organ. As liver cells die, they are replaced with scar tissue, a process known as fibrosis.

Related: Scientists discover new type of cell in the liver

There are three types of PFIC — PFIC1, PFIC2 and PFIC3 — which differ in that they are caused by mutations in different genes that code for proteins needed for the liver to function properly. PFIC is inherited in an autosomal recessive manner, meaning that children must inherit two copies of a relevant mutated gene — one from each parent — to develop the disease.

Symptoms: All patients with PFIC develop symptoms of liver disease, which typically appear during infancy. These symptoms include severe itching; a yellowing of the skin and whites of the eyes, known as jaundice; stunted growth; and high blood pressure in the vein that carries blood from the digestive system to the liver. The itchiness stems from excess bile acid irritating nerve cells in the body.

Patients with PFIC1 also may have other symptoms, such as short stature, deafness, diarrhea and inflammation of the pancreas. And those with PFIC2 are at an increased risk of developing a type of liver cancer called hepatocellular carcinoma.

Symptoms of liver failure usually develop before adulthood in patients with PFIC1. The prognosis is typically worse for patients with PFIC2 whose livers fail within the first few years of life. Patients with PFIC3, on the other hand, may develop liver failure in childhood or adulthood.

Treatments: There is no cure for PFIC, but steps can be taken to ease patients' symptoms. For instance, certain drugs can increase the flow of bile out of the liver, while dietary supplements can raise low levels of vitamins and fats in the blood caused by the lack of bile secretion.

Even with these treatments, though, most patients with PFIC will ultimately require a liver transplant. This replaces damaged liver tissue with healthy cells that do not have the genetic defect, thus reversing many symptoms of liver disease.

If left untreated, PFIC kills an estimated 87% of patients. By comparison, several studies have shown that survival rates after liver transplantation range between 76% and 85%; one of these studies followed patients for 19 years after the procedure.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Mucormycosis, also known as "black fungus"

Affected populations: Mucormycosis is a potentially fatal fungal infection that typically affects people with weakened immune systems, such as those with diabetes or severe COVID-19, including people in recovery from the infection. It can also affect individuals who have received a solid organ transplant or who have a low number of white blood cells, a type of immune cell that normally fights infections. People with HIV and those who use immunomodulating drugs are also at high risk of developing mucormycosis.

The World Health Organization estimates that cases of mucormycosis range between 0.005 and 1.7 per million people worldwide. The burden of the disease is much higher in specific countries, such as India, where there are approximately 80 times more cases at any point in time, potentially influenced by climatic factors and the dominance of certain fungal species in the environment.

The exact incidence of mucormycosis in the United States is unknown; because the condition is rare, there is no national surveillance program to track the disease. However, a study in San Francisco in the late 1990s estimated that there may be 1.7 new cases of mucormycosis per million people every year.

Related: 'Black fungus' treatment runs short in India as new cases of infection emerge

Causes: Mucormycosis is caused by a group of fungi known as mucormycetes — in particular, species in the genus Rhizopus. Mucormycete spores are found naturally in the environment, including in soil, fallen leaves, compost, animal dung and the air. People can get mucormycosis after inhaling or ingesting these spores or if the spores accidentally enter the body via a cut or burn.

Mucormycete spores live harmlessly in many people's bodies, and they typically cause infection only in some individuals who have weakened immune systems. Most cases of mucormycosis occur randomly — in other words, they're not linked to a common infection source. However, outbreaks of the disease have been reported; for instance, health care settings can see outbreaks if fungal spores contaminate hospital supplies or ventilation systems.

Once inside the body of a vulnerable person, mucormycete fungi invade the blood vessels, causing blood clots to form and tissues to become deprived of nutrients and oxygen and die.

Symptoms: Symptoms of mucormycosis differ depending on the part of the body that is affected. These body parts can include the sinuses, lungs, skin, stomach and intestines.

Most mucormycosis infections tend to occur in the sinuses or the lungs after a person inhales fungal spores from the air, which then triggers symptoms such as facial swelling, headaches, fever, cough and chest pain. Mucormycosis infection can also spread from the respiratory system to other parts of the body, such as the brain, spleen and heart.

Overall, death rates for mucormycosis usually exceed 50%, even with medical care.

Mucormycosis is not contagious — people can pick up the spores from the environment, but they cannot then spread those spores to others. In other words, the disease cannot be transmitted from one person to another.

Treatments: It is crucial to diagnose mucormycosis and begin treatment as early as possible. This treatment involves taking antifungal drugs — usually amphotericin B — to kill the fungi that cause the infection, as well as surgically removing any dead tissue to help prevent spores from spreading to other regions of the body.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Norrie disease

Affected populations: Norrie disease is an extremely rare, inherited genetic disorder that causes vision loss and was first described in Denmark in 1927. The exact incidence of Norrie disease in the population is unknown, but around 500 cases have been reported worldwide. The disease mainly affects males, and when females do develop it, they tend to experience milder symptoms.

Causes: Norrie disease is caused by more than 100 different types of mutation in a gene called Norrin cysteine knot growth factor (NDP). The gene codes for a protein called norrin, which is essential for cell and tissue development and is believed to help protect neurons in the retina, the light-sensitive tissue at the back of the eye. The protein also plays a role in the formation of blood vessels in this area of the eye and in the inner ear.

The NDP gene is found on the X chromosome — otherwise known as the female sex chromosome. (Human females typically have two X chromosomes (XX) — one of which is inherited from each parent — while males have an X and a Y (XY), the latter being the male sex chromosome.)

Related: Weird 'gut-eye axis' links the retina and intestines, and may help explain glaucoma

Norrie disease is inherited in a recessive manner, meaning that people have to inherit two copies of the mutated NDP gene — one from each parent — to develop the condition. However, because males typically inherit just one X chromosome, if that chromosome contains the faulty gene, they will definitely develop the condition. Females, meanwhile, may be "protected" by having a second, working copy of the gene on their other X chromosome. This likely explains why most patients with Norrie disease are male, and affected females tend to have milder symptoms.

Symptoms: The main symptom of Norrie disease is vision loss. As a result of mutations in NDP, the retinas don't form properly and then detach from the backs of the eyes, leading to complete blindness in both eyes. This usually happens at birth or within a few months of birth.

An ophthalmologist may also detect other signs of the condition while conducting an eye exam, such as widened or white pupils, as well as bleeding in the center of the eye or clouding of the lens of the eye, known as cataracts. Some of these symptoms may be visible at birth, but others may develop months or years later.

Beyond vision loss, approximately 30% of patients with Norrie disease also develop progressive hearing loss because of the condition — usually around the age of 12 — and between 30% and 50% experience developmental delays in motor skills, including sitting up and walking.

There are also behavioral symptoms associated with Norrie disease. For instance, around 1 in 4 patients may have episodes of uncontrollable or inappropriate laughing or crying. A similar percentage of patients with Norrie disease also have autism, and approximately 1 in 10 may experience seizures.

Treatments: There are no clinical treatments to prevent the hearing loss or blindness associated with Norrie disease. However, scientists are trying to develop new gene therapies to treat the disorder, namely by replacing the faulty NDP gene with a functional copy. For example, a 2023 study found that gene therapy could stop hearing loss associated with Norrie disease in mice.

In the meantime, cochlear implants can significantly improve the quality of life of the patients with hearing loss. Patients may also benefit from using hearing aids, or undergoing eye surgery to help preserve some of their vision at the earlier stages of the disease before it is lost.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Creutzfeldt-Jakob disease (CJD), named after Hans Creutzfeldt and Alfons Jakob, two German doctors who first described the disease in the 1920s.

Affected populations: CJD affects around 1 in a million people worldwide each year. In the U.S., approximately 350 cases of CJD are diagnosed annually. Males and females are equally likely to develop the disease.

Causes: CJD is caused by abnormal proteins in the brain known as "prions." These prions cause irreversible damage to tissues, leading to the formation of sponge-like holes throughout the brain that cause neurons to malfunction and die. Prions spark a chain reaction, prompting other, normal proteins in the brain to misfold. This perpetuates the condition and causes patients to experience progressively worse issues with movement and mental function.

Related: Lab tech develops fatal brain condition after accident with 'mad cow disease' samples

Three main types of CJD exist, which differ depending on how the prions originate in the brain. The most common of these forms is "sporadic CJD," which accounts for around 85% of cases. Sporadic CJD occurs when normal proteins spontaneously misfold and become prions for unknown reasons, with symptoms usually first developing in adults who are between 45 and 75 years old.

Additionally, between 10% and 15% of cases of CJD are caused by a mutation in a gene called PRNP, which leads to the development of prions. This genetic form of CJD is inherited in an autosomal dominant manner, meaning that a child only needs to inherit one copy of the faulty gene from either parent to develop the condition. Genetic CJD most often occurs in people between ages 30 and 50.

Fewer than 1% of CJD cases are "infectious," meaning they are triggered by the transmission of prions from external sources. One way this can happen is when people eat beef from cows that have bovine spongiform encephalopathy, better known as "mad cow disease." The U.S. has had strict regulations in place since the 1990s to prevent this from happening. Since the 1996 discovery that people could get CJD from "mad cows," only 233 such cases have been reported worldwide.

Another way CJD can be transmitted is when it's accidentally passed from human to human during medical procedures — for instance, if a patient receives a transplant or blood transfusion from a donor with CJD. A prominent example of this happened from the late 1950s to 1985, when doctors gave patients contaminated growth hormones derived from corpses. This led to at least 226 cases of CJD worldwide, including 29 cases in the U.S.

Symptoms: Common symptoms of CJD include dementia, confusion and disorientation, hallucinations, lack of coordination and muscle stiffness. Patients may also experience personality changes, sleepiness, seizures and issues speaking. They can additionally have psychological symptoms, such as severe depression, anxiety and irritability.

Symptoms of CJD often quickly progress to the point where patients become completely bedridden, unaware of their surroundings and unable to communicate with those around them. CJD is always fatal, and approximately 70% of patients die within a year of diagnosis, most often because of an infection, heart or lung failure.

Treatments: There is no cure for CJD, but drugs can help alleviate a patient's symptoms. For instance, patients may be prescribed medications to reduce muscle jerking or to alleviate anxiety.

Early diagnosis of the genetic form of CJD can help patients by enabling them to make arrangements for end of life care and family planning.

Recent cases: In April 2025, public health officials reported three cases of CJD detected in individuals in Hood River County in Oregon within the past eight months — two of whom have subsequently died. As of yet, no link between the cases has been identified.

Editor's note: This article was updated on April 16, 2025 to include information on the recent cases in Oregon. The article was originally published on Feb. 13, 2025.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: DNA ligase IV (LIG4) syndrome

Affected populations: LIG4 syndrome is an extremely rare inherited condition that was first reported in the U.K. in 1990. Little is known about its exact prevalence worldwide, but as of 2020, approximately 55 cases had been described in the medical literature.

Causes: LIG4 syndrome is caused by a mutation in the LIG4 gene, which carries the instructions needed to make a protein known as DNA ligase 4. This protein is an enzyme that helps repair a specific type of DNA damage —- namely, breaks in both sides of the DNA double helix structure.

These kinds of breaks are commonplace, occurring between 10 and 50 times a day in the average cell in the human body. The breaks are caused by a variety of things, including normal cellular processes like DNA replication, which is needed for cells to multiply, as well as external factors, like exposure to certain chemicals or radiation.

Related: New study provides first evidence of non-random mutations in DNA

If these DNA breaks aren't repaired, they can prompt cells to self-destruct, or failing that, become cancerous. As LIG4 syndrome impairs this response, people with the condition are particularly susceptible to the knock-on effects of radiation.

The enzyme DNA ligase 4 is also needed to make vital proteins on the surface of immune cells called T cells and B cells that help them to work properly and produce antibodies that fight off infections. Hence, people with LIG4 may also develop an immunodeficiency disorder, such as severe combined immunodeficiency, as a result of their condition.

LIG4 syndrome is inherited in an autosomal recessive manner, meaning that a child needs to inherit two copies of the faulty LIG4 gene — one from each parent — in order to develop the condition.

Symptoms: People with LIG4 syndrome typically experience an array of symptoms, which may either develop soon after birth or later in life. These symptoms include microcephaly, when a baby's brain is much smaller than expected; delays in growth and development; and a reduction in the amount of cells in the blood, including immune cells that help the body to fight infections.

Other characteristic symptoms of LIG4 syndrome are a "bird-like" appearance of the face, as well as skin lesions. People may also have an abnormally-shaped skeleton and develop progressive failure of the bone marrow; bone marrow is a key site of antibody and blood-cell production, so this can cause widespread problems.

Treatments: There is currently no cure for LIG4 syndrome.

However, patients may be offered treatments to help lower their risk of developing severe infections as a result of their immunodeficiency. For example, they may be prescribed antiviral and antifungal drugs or antibiotics.

People with LIG4 syndrome may also receive injections of antibodies to replace defunct ones and they are often advised to avoid any unnecessary exposure to radiation, such as X-rays produced by medical equipment, to minimize potential DNA damage.

According to medical case reports, 10 people with LIG4 syndrome have also reportedly been given a bone marrow transplant in a bid to treat their condition by replenishing their stocks of immune cells. This procedure was successful for six of the 10 individuals. However, the remaining four still died despite treatment — mainly because of infections.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Alkaptonuria, also known as "black urine disease"

Affected populations: The exact prevalence of alkaptonuria worldwide is unknown. However, estimates suggest that the disease affects between 1 in 250,000 and 1 in 1 million people in the U.S. More than 1,000 cases of alkaptonuria have been reported in the medical literature. Individuals from any group can potentially develop the disease, regardless of their sex, race or other factors. However, alkaptonuria is a recessive genetic disorder, meaning people will only develop it if they inherit two faulty copies of a specific gene — one from each of their parents.

Causes: People with alkaptonuria are unable to fully break down all the building blocks of proteins that they consume or that their body makes. This is because a mutation in a gene called homogentisate 1,2-diooxygenase (HGD) causes an enzyme that normally does the job to malfunction. Consequently, a chemical called homogentisic acid accumulates in different tissues in the body, especially in connective tissues, such as cartilage.

Related: Finally, we know why pee is yellow

Symptoms: One of the earliest signs that someone has alkaptonuria is dark stains in their diapers as babies, because the homogentisic acid in their urine turns black upon exposure to air.

Besides dark urine, people with alkaptonuria often have no other noticeable symptoms of the disease until they are in their late 20s or early 30s.

Over time, homogentisic acid continues to build up in a person's tissues, staining those tissues a slate blue or black color and eventually weakening and damaging them. This can lead to severe deformities in the joints and spine, as well as organ dysfunction, including issues with the kidneys and heart. Symptoms of alkaptonuria tend to be more severe and come on sooner in males than in females.

Although patients with alkaptonuria tend to have a normal life expectancy, the disease can markedly impact their quality of life.

Treatments: There is currently no cure for alkaptonuria, but scientists are studying the potential use of a drug called nitisinone to slow the progression of the disease. Affected individuals can also take steps to manage their symptoms, including taking painkillers and exercising regularly to help strengthen their joints. Adopting a low-protein diet can also help some patients.

Around half of patients with alkaptonuria will require hip, knee or shoulder joint replacement, usually by the age of 50 or 60.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Capnocytophaga

Affected populations: Capnocytophaga is an extremely rare condition — two large epidemiological surveys conducted in Scandinavian countries suggest that it may affect between 0.5 and 0.7 people out of every million each year. In the United States, the disease is not nationally notifiable, which means it's not tracked at a national level and so there are no annual estimates of its incidence. As of 2018, approximately 500 cases of Capnocytophaga infection caused by the bacterial species Capnocytophaga canimorsus had been reported in the medical literature.

People with weakened immune systems are at a higher risk of catching Capnocytophaga and developing serious illness. Other risk factors include not having a spleen, having alcohol use disorder, smoking and being over age 40.

Causes: Capnocytophaga infection can be caused by nine species of bacteria that belong to the same genus: Capnocytophaga. Some of these species, such as Capnocytophaga gingivalis, naturally live in humans' mouths and tend to cause infections mostly in people who have weakened immune systems.

However, most Capnocytophaga infections in humans are caused by C. canimorsus, a species that lives in the mouths of dogs and cats.

Related: A woman needed her hands and legs amputated after contracting infection from dog 'kisses'

Patients who become infected with C. canimorsus usually get it through the bite of a dog or cat, or if dog or cat saliva enters an open wound or sore on the person's body. However, most dog or cat bites do not cause Capnocytophaga or any other illness. In the U.S., for instance, around 4.5 million people are bitten by dogs every year, but only 20% of them require medical attention for resulting injuries or infections.

Symptoms: In people who do develop Capnocytophaga infection, symptoms normally begin between three and five days after a dog or cat bite or scratch. Patients may develop blisters at the wound site, as well as redness, swelling and pain. They may also experience fever, diarrhea, vomiting and headaches.

Capnocytophaga bacteria can also spread beyond the wound to other organs, triggering serious conditions such as heart attacks, kidney failure and gangrene, the death of bodily tissue. Infection may additionally lead to septicemia (blood poisoning) or meningitis (infection of the membranes over the brain and spinal cord). Patients who develop these serious conditions have a 30% or 5% chance of survival, respectively.

Patients who lack a spleen have between a 30% and 60% greater risk of death from Capnocytophaga infections than people who have the organ, and they can die within one to three days of their symptoms beginning. These patients may be particularly susceptible because the spleen plays a vital role in the immune system's response to bacteria like Capnocytophaga.

Treatments: Capnocytophaga infection can be treated with antibiotics, and starting the therapy quickly can lower the risk of developing serious complications. Patients who develop gangrene may have to have limbs amputated.

The Centers for Disease Control and Prevention recommends that anyone who is bitten by a dog or cat call their health care provider immediately or go to an urgent care clinic to be assessed. They should also wash the bite area with soap and water right away, the agency advises.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Argyria

Affected populations: Although more common in the 19th century because of heavy exposure to silver among people who worked with the metal and the use of silver to treat diseases, argyria is now a rare condition. Consequently, there are limited statistics on the true incidence of the disease. However, one review of medical literature published up to 2018 identified 16 patients who developed argyria after exposure to a liquid suspension of silver, known as colloidal silver.

Causes: Argyria is caused by the ingestion of — or exposure to — silver or silver-containing compounds. The silver accumulates in the body and thus turns the skin and nails bluish-gray. The human body normally contains tiny amounts of silver from more minor exposures, such as from food and drinking water. But in patients with argyria, these levels have built up in excess over a long period of time, ranging from several months to years.

People who work with silver or use colloidal silver as a dietary supplement are at the greatest risk of developing argyria. This includes people who consume colloidal silver as a form of alternative medicine in an attempt to treat conditions such as high blood pressure and diabetes. U.S. health officials have warned that the compound isn't effective for treating any condition and that it can interfere with certain drugs, such as antibiotics.

Related: Butterfly disease: A disorder that makes skin as delicate as butterfly wings

Symptoms: The main symptoms of argyria are the skin developing a bluish-gray or gray tint, the fingernails growing darker than normal, and darker patches appearing on skin in areas that are exposed to the sun — a condition known as hyperpigmentation. These color changes are typically permanent, and patients with argyria may experience psychological distress because of their appearance.

Symptoms can vary depending on how much silver enters a person's body, how it enters the body, and how long it has accumulated. Symptoms may occur across the entire body (generalized argyria), only select parts of the body (localized argyria) or specifically in the eye (argyrosis).

Argyria can also cause other symptoms — such as abdominal pain, fatigue, headaches and kidney damage — but it is not life-threatening. That said, ingesting large amounts of silver in one go can be fatal.

Treatments: There is currently no cure for argyria. Laser therapy has been tested as a means to improve the appearance of discolored skin in patients with argyria in several studies, without success.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Tularemia, also known as "rabbit fever" or "deer fly fever"

Affected populations: This disease is rare in the U.S. Between 2011 and 2022, 2,462 cases of tularemia were reported in 47 states, according to the Centers for Disease Control and Prevention (CDC).

Historically, human tularemia infections have been reported in every U.S. state except Hawaii. They are especially likely to occur in rural areas of Arkansas, Missouri, Oklahoma and Kansas, and they usually happen between May and September. Most cases occur in children, especially males.

Causes: Tularemia is an extremely infectious disease caused by Francisella tularensis bacteria. The microbe is found throughout the Northern Hemisphere and occasionally in the tropics and Southern Hemisphere.

Related: 'Unusual' beaver die-off in Utah caused by 'rabbit fever,' which can also infect humans

There are four types, or subspecies, of F. tularensis, which differ in terms of their location and their propensity to cause severe disease. F. tularensis type A, for instance, is the most dangerous type and is only found in North America.

As its nickname suggests, tularemia normally affects animals such as rabbits, hares and rodents, but humans can also become infected. This can happen in several ways: through the bite of infected ticks or deer flies; by drinking contaminated water; or via physical contact with an infected animal, including being bitten. As few as 10 to 25 individual bacterial cells can cause tularemia infection in humans.

Tularemia cannot be spread from person to person. People who participate in activities such as hunting, wildlife management, hiking and camping are at higher risk of tularemia than the average person.

Symptoms: The exact symptoms of tularemia in humans depend on where the bacteria enter the body, but infected individuals usually experience a fever up to 104 degrees Fahrenheit (40 degrees Celsius).

If F. tularensis enters the body via the skin, infected people may also develop an ulcer at the site of infection, as well as swelling of their lymph nodes, particularly in the armpit or groin. People who eat or drink food or water contaminated with F. tularensis may develop a sore throat, mouth ulcers and tonsilitis, or inflammation of the tonsils.

In the most serious cases of tularemia, in which people inhale dust or aerosols containing F. tularensis, the disease may cause symptoms in the lungs, including chest pain, a cough and breathing difficulties. These symptoms can also arise if an F. tularensis infection in other parts of the body is not treated and the bacteria then spread to the lungs.

Treatments: Tularemia can be treated with broad-spectrum antibiotics, meaning those that are capable of killing a wide variety of bacteria. This treatment can lower the death rate of the disease from between 5% and 15% to 2%. There is currently no vaccine against tularemia that is approved for use in the U.S.

People can take precautions to prevent tularemia, according to the CDC. These include using insect repellant while outdoors and wearing gloves when handling sick or dead animals.

This article is for informational purposes only and is not meant to offer medical advice.

Disease name: Situs inversus

Affected populations: Approximately 1 in 10,000 people have situs inversus. Men are 1.5 times more likely than women to experience the condition.

Causes: Situs inversus is a genetic condition that causes organs in the chest and abdomen, such as the liver and spleen, to be flipped across the midline of the body. If the heart's position is also flipped — a phenomenon known as dextrocardia — then the condition is described as situs inversus totalis.

Situs inversus is caused by mutations in one or more genes. More than 100 genes have been tied to defects in the "sidedness" of the body, such as the gene NME7, which encodes a protein that helps regulate the creation of tube-like cellular structures, called microtubules. These structures have numerous functions, including supporting cell shape and their ability to move around the body.

Related: 10 body parts that are useless in humans (or maybe not)

Situs inversus is inherited in an autosomal recessive manner, meaning that both parents need to pass on a mutated gene for their children to develop it.

People may have situs inversus on its own, or alongside another condition such as primary ciliary dyskinesia. This is when there's dysfunction in cilia — the tiny, wiggling hair-like structures that keep the airways, ears and sinuses free from infectious germs. During embryonic development, cilia help determine the left-right axis of the body.

Symptoms: Many people with situs inversus will not develop any notable symptoms from the condition — although the positions of their organs are flipped, the organs still work normally. Many individuals may not even realize that they have situs inversus until they are examined by doctors for another, unrelated condition.

However, between 5% and 10% of people with situs inversus have heart defects that are present from birth. And those who also have primary ciliary dyskinesia may be more likely to develop respiratory infections, such as bronchitis and sinusitis, in which the airways and the sinuses are inflamed, respectively.

If they are unaware of their condition, people with situs inversus may also be at risk of diagnostic errors if they're hospitalized for certain ailments. For instance, appendicitis typically causes pain on the lower right-hand side of the abdomen, where the appendix is normally located. But this condition may be missed or diagnosed later than usual if the pain a person experiences is on their left-hand-side instead.

Treatments: As situs inversus doesn't usually cause any symptoms, there is generally no need for treatment. Instead, treatment is focused on targeting the symptoms of any co-occurring conditions, should any emerge.

This article is for informational purposes only and is not meant to offer medical advice.

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Disease name: Frosted branch angiitis (FBA)

Affected populations: FBA is a rare disease that most often affects young and healthy people without other notable conditions, with most cases occurring in people ages 2 to 42. Around 61% of patients with FBA are male. The disease has been reported worldwide, including in the United States, Europe and Asia.

Causes: FBA was first described in 1976 in a 6-year-old Japanese boy who had inflammation in the inner structures of his eye, as well as in the blood vessels that supply the retina, the light-sensitive tissue at the back of the eye. When doctors examined the retina, the affected blood vessels looked a bit like the frosted branches of a tree, hence the name of the disease.

FBA can arise in several ways. For instance, it can occur spontaneously in otherwise healthy individuals following a viral illness, or it can emerge in patients who have a chronic infection, including with cytomegalovirus or tuberculosis, or an inflammatory disease such as Crohn's disease or lupus. FBA is thought to be caused by the immune system going into overdrive, which triggers excessive inflammation that damages eye tissue.

Related: Weird 'gut-eye axis' links the retina and intestines, and may help explain glaucoma

Symptoms: Patients with FBA usually have severe levels of inflammation that causes a white "sheath" to form around the blood vessels that supply the retina, which then swells and causes patients to lose their sight. Around 55% of patients with FBA develop the condition in both eyes.

Treatments: According to case records, doctors have historically treated FBA using anti-inflammatory drugs, such as steroids, as well as by directly targeting the potential cause of the inflammation. For instance, they may use antivirals or antibiotics to treat an underlying infection. Patients with FBA tend to recover well and rarely experience the condition more than once in their lives. Complications that require surgery are not very common.

This article is for informational purposes only and is not meant to offer medical advice.

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Disease name: Fatal familial insomnia (FFI)

Affected populations: The disease affects an estimated 1 to 2 people per million every year, according to the National Organization of Rare Disorders. FFI is passed from parent to child, and between 50 and 70 families worldwide are believed to carry the genetic mutation that causes FFI. Males and females are equally likely to develop the condition.

Causes: FFI is a neurodegenerative prion disease that is caused by a mutation in a gene called PRNP, which produces a so-called prion protein. Prions are misfolded versions of normal proteins, and their abnormal shape is toxic to cells in the body, particularly neurons in the brain. One of the tissues that is primarily damaged in patients with FFI is the thalamus, a region of the brain that regulates an array of body functions including sleep, body temperature and appetite.

Children need to inherit only one copy of the mutant PRNP gene from a parent to develop the condition. In rare instances, patients may spontaneously develop mutations in the PRNP gene, despite having no family history of FFI. They can then pass this mutation on to their children in the regular way.

Related: Not all insomnia is the same — in fact, there may be 5 types

Symptoms: The hallmark symptom of FFI is insomnia, or the inability to fall or stay asleep, which progressively worsens over time to the point where patients cannot sleep at all.

Patients with FFI also commonly experience memory loss, high blood pressure, hallucinations and involuntary jerking of their muscles. They may sweat profusely and lose their coordination.

Symptoms usually begin around age 40, but can develop as early as age 20 or as late as age 70. Patients eventually enter a coma-like state and typically die within nine to 30 months after their symptoms emerge.

Treatments: There is currently no cure for FFI. As the disease is so rare, there is also no standard way of treating it. Instead, patients may be given advice on how to best manage their symptoms and live as comfortable a life as possible. For instance, taking the drug clonazepam can reduce muscle jerking.

A medical case report from 2006 showed that trying to induce sleep — for instance, by engaging in rigorous exercise and taking narcoleptic drugs — extended and enhanced the life of 52-year-old man with FFI by about a year, but did not prevent his death.

In 2015, a clinical trial of a drug that aims to prevent the onset of FFI was launched. Over 10 years, 10 people who carry the FFI mutation will be given the antibiotic doxycycline and their prognosis and survival after disease onset will be compared to patients who previously died of FFI. Doxycycline has also been shown to prevent the formation of misfolded proteins in another prion disease, called Creutzfeldt-Jakob disease, helping patients live twice as long as those who didn't receive the treatment.

This article is for informational purposes only and is not meant to offer medical advice.

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Disease name: Angelman syndrome

Affected populations: The disorder is believed to affect somewhere between 1 in 12,000 and 1 in 24,000 people, although these figures may be underestimated. Many cases of Angelman syndrome can go undiagnosed because the disorder shares symptoms and characteristics with other conditions. Men and women are equally likely to experience the disorder.

Causes: Angelman syndrome is a genetic disorder named after Dr. Harry Angelman, who first reported it in 1965. The disorder principally affects the central nervous system, meaning the brain and spinal cord, and it's caused by mutations in the UBE3A gene. This gene carries instructions for a protein needed to maintain the normal development and function of cells, including neurons in the brain.

In each cell, humans typically have 23 pairs of chromosomes — thread-like structures that house DNA. One parent contributes half of each chromosome pair, and the other parent contributes the other half. UBE3A is located on chromosome 15. Mutations that delete the gene or change its structure, function or activity can cause Angelman syndrome. In many cases, a mutation develops spontaneously on the copy of the gene from the mother.

The genetic mutations behind Angelman syndrome usually occur randomly, but between 3% and 5% of children inherit them from their parents. In around 10% of people with the condition, the exact cause of the syndrome can't be identified.

Related: New genetic cause of intellectual disability potentially uncovered in 'junk DNA'

Symptoms: People with Angelman syndrome normally start to develop symptoms of the disorder in early childhood. These include developmental problems, such as having difficulty sitting unsupported or babbling, that become apparent around ages 6 to 12 months.

As the disorder progresses, affected people may struggle to speak and walk because of balance and coordination issues. They may also experience seizures, which usually begin when a child is between 2 and 3 years old. Furthermore, some people with the condition may have distinctive facial features, such as a prominent chin, deep-set eyes or abnormally wide mouth. People who have the syndrome usually have a normal life expectancy.

Symptoms of Angelman syndrome can sometimes be confused with other disorders that also cause developmental delays, such as autism or cerebral palsy, possibly leading to misdiagnosis.

Treatments: There is currently no cure for Angelman syndrome.

However, several treatment options exist to help manage their symptoms. For example, doctors may prescribe anti-epileptic drugs to control patients' seizures. Physiotherapy and communication therapy can also help to respectively improve patients' ability to walk and communicate with others without speaking — by using hand gestures or signs, for example.

This article is for informational purposes only and is not meant to offer medical advice.

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Affected populations: Acromegaly is estimated to affect approximately 50 to 70 people out of every million. However, the true figure may be higher, as the symptoms develop slowly and thus may go unrecognized. Males and females are equally likely to be affected by the disorder.

Causes: Acromegaly occurs when the pituitary gland, a small hormone-making structure in the brain, produces too much growth hormone for a long period of time. This hormone normally regulates the physical growth of the body, including the growth of bones and muscles.

In most cases of acromegaly, this excessive growth hormone production stems from a noncancerous tumor in the pituitary gland. However, in rarer instances, the disease can be triggered by tumors elsewhere in the brain or in other parts of the body, such as the lungs or pancreas. These tumors either produce growth hormone themselves or make another substance, known as growth-hormone-releasing hormone, which, in turn, prompts the pituitary gland to make more growth hormone.

Some genetic conditions can cause acromegaly. For instance, multiple endocrine neoplasia type 1 and Carney complex are two rare disorders that increase people's risk of developing tumors in hormone-producing glands.

Related: Have giant humans ever existed?

Symptoms: Symptoms of acromegaly usually begin sometime after puberty, most commonly between the ages of 40 and 50. The disease causes a person's bones to enlarge — most notably, in the hands, face and feet. Characteristic features in the face include a disproportionately large nose, tongue or jaw, and lips. Elsewhere in the body, patients may develop thicker body hair and skin, experience joint pain, and sweat more than usual. They also may experience headaches, a loss of vision and irregular periods in those who menstruate. Other organs, such as the heart, may also enlarge.

If left untreated, acromegaly can cause a variety of health issues, including type 2 diabetes, high blood pressure, sleep apnea (when people temporarily stop breathing during sleep) and heart disease. It may also reduce someone's life expectancy by around 10 years.

Acromegaly is often confused with gigantism, another rare condition that causes excessive growth as a result of high levels of growth hormone production. However, unlike acromegaly, gigantism begins during childhood.

Treatments: There are several treatment options for acromegaly, depending on the size and location of the tumor behind the disease, the severity of a patient's symptoms, their age and their general health. Surgery and radiation therapy can be used to remove or shrink a tumor, while drugs can help reduce the levels of growth hormone that circulates in the body or stop the hormone from exerting its effects on tissues.

Acromegaly is curable in some cases. In patients who have a small pituitary gland tumor that can be removed via surgery, the disease can be cured 85% of the time. For patients with large-but-removable tumors, it can be cured between 40% and 50% of the time. Unlike surgery, drugs can't cure the condition, but they can help control patients' symptoms.

This article is for informational purposes only and is not meant to offer medical advice.

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Disease name: Fibrodysplasia ossificans progressiva (FOP), also known as "stone man disease" or "Münchmeyer disease"

Affected populations: FOP is an extremely rare disease that is estimated to affect around 1 in 1 million people. It does not affect one demographic more than others; anyone may develop FOP, regardless of their sex, race or ethnicity.

Causes: FOP is caused by a mutation in a gene called ACVR1, which is involved in a cellular process that regulates the formation of the skeleton in the womb and then orchestrates skeletal repairs throughout life. The mutation causes the ACVR1 gene to become more active, leading to inappropriate bone formation.

In most cases, this mutation occurs randomly in people who have no family history of the disease. However, in rarer instances, patients may inherit the mutation from one parent who is affected by the condition. Only one copy of the mutated gene is required to cause FOP.

Related: Céline Dion's muscle spasms caused by rare disorder: What is stiff-person syndrome?

Symptoms: FOP causes the body's muscle and connective tissue to be gradually replaced with bone tissue. This essentially forms a second skeleton that impedes the person's movement. One of the first signs that someone has FOP is that they have shortened and turned-in big toes from birth. Around 50% of patients will also have similarly malformed thumbs.

Symptoms of FOP usually start during childhood, with some of the earliest affected areas normally being the neck, back, chest, arms and legs. Patients may also experience "flare-ups," which are episodes of tissue swelling, pain and stiffness that worsen the condition. These flare-ups can occur spontaneously, or they may be triggered by some sort of trauma, such as an injury, surgery or a viral infection, like the flu. Other symptoms of FOP include difficult speaking or eating (if the mouth is affected and cannot open properly), as well as hearing loss and spinal deformity.

By age 30, most patients with FOP are completely unable to move. The average life expectancy of people with the disease is around 56 years. The most common cause of death is cardiorespiratory failure due to patients losing the ability to breathe.

Treatments: Painkillers and anti-inflammatory drugs, such as corticosteroids, can help manage patients' symptoms. Patients may also be advised to take extra precautions to minimize the risk of injury and infection, and they may engage in occupational therapy to help them carry out daily activities.

Doctors can't conduct surgery to remove excess bone tissue, as this could inadvertently trigger further flare-ups of the condition.

In 2023, the U.S. Food and Drug Administration approved an oral drug called palovarotene (brand name: Sohonos) for the treatment of FOP in females 8 years and older and males 10 years and older. This approval followed a clinical trial in 107 patients with the condition. Those treated with the drug showed a reduction in excessive bone formation — around 54% lower than controls.

This article is for informational purposes only and is not meant to offer medical advice.

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Disease name: Epidermolysis bullosa (EB), or "butterfly disease"

Affected populations: Butterfly disease is estimated to affect around 1 in 50,000 children, if you count all subtypes of the disease together. EB is equally common among males and females, as well as across races and ethnicities.

Causes: Butterfly disease refers to a group of rare, inherited diseases that cause the skin to be very fragile and to blister easily. There are around 30 subtypes of butterfly disease, and these are sorted into four main groups that differ based on the part of the skin that is affected.

The most common form of the disease, which affects 70% of EB patients, is known as epidermolysis bullosa simplex (EBS). Patients with EBS carry genetic mutations that affect the outermost layer of their skin, called the epidermis. EBS is usually an autosomal dominant disorder, meaning that children who inherit just one copy of the mutant gene from a parent will develop it. Rarely, the disease passes in an autosomal recessive pattern, in which a child needs two copies of the gene — one from each parent — to develop it.

Related: 'Butterfly disease' makes the skin incredibly fragile, but a new gene therapy helps it heal

Symptoms: Patients usually begin to develop symptoms of butterfly disease at birth or during early childhood. Symptoms can vary from mild to severe, and they differ across the subtypes of the disease. Common, universal symptoms of EB include having skin that blisters easily, especially on the hands and soles of the feet. These areas of the body also often have thickened, scarred skin. Mild forms of butterfly disease can improve with age and aren't fatal, but patients with severe forms typically don't live beyond age 30. That's because severe cases can lead to life-threatening infections and damage internal organs.

Treatments: There is currently no cure for butterfly disease. However, appropriate wound care — for instance, draining blisters and using nonadhesive bandages and dressings to cover wounds — can help patients manage their symptoms. Drugs can also be taken to relieve the itching and pain associated with the blisters, and antibiotics can treat any related bacterial infections. Some patients may also require surgery if the disease causes their esophagus to constrict or if they have problems using their hands due to excessive scarring.

In 2023, the U.S. Food and Drug Administration (FDA) approved a new gene therapy gel for the treatment of a specific type of butterfly disease in patients who are at least 6 months old. Called dystrophic epidermolysis bullosa, this form of the disease is caused by mutations in a gene that codes for a type of collagen in the skin. The gene therapy, known as Vyjuvek, delivers working copies of the collagen gene into patients' cells.

This same gene therapy was also adapted into eye drops in 2023. It was used to help restore the vision of a teenage boy who was legally blind due to scarring of the eyes caused by butterfly disease.

The FDA has also approved another gel, called Filsuvez, for the treatment of wounds in patients 6 months and older. The gel, made from birch bark, is approved for only certain types of butterfly disease.

This article is for informational purposes only and is not meant to offer medical advice.

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Disease name: Necrotizing fasciitis, also known as "flesh-eating disease"

Affected populations: Flesh-eating disease can affect anyone. It occurs in about 0.4 in 100,000 people in the U.S. each year. The disease is most commonly seen in people with conditions that weaken the immune system, such as cancer, liver scarring (cirrhosis), diabetes and kidney disease.

Causes: Necrotizing fasciitis is caused by one or more bacteria infecting the fascia, the connective tissue that runs below the skin. Fascia also surrounds organs of the body, helping to hold them in place. Often, flesh-eating disease is caused by Group A streptococci bacteria, such as Streptococcus pyogenes. But other bacteria — like Staphylococcus aureus, Klebsiella and Escherichia coli — can trigger the infection.

These bacteria get into the connective tissue, either through an external injury or from a ruptured organ. They then release substances that restrict the blood supply to the infected tissue and begin "eating," or digesting the cells within it. This causes pus-filled lesions containing dead cells to form. If treatment is delayed, the infection can trigger deadly conditions such as sepsis, organ failure or toxic shock syndrome.

Related: Decades-long mystery of flesh-eating, ulcer-causing bacteria solved

Symptoms: Early symptoms of flesh-eating disease often resemble those of the flu, such as body aches, fever, chills and nausea. Patients may also experience intense pain and swelling near an infected cut or wound, if that's how the bacteria entered their body. Symptoms can then quickly progress within hours or over a few days as pus-filled blisters begin to form at the wound site and tissue is destroyed, causing it to turn black. The disease can also cause delirium, extremely low blood pressure, and a rapid heart rate. In late stages of the disease, a patient's vital organs may shut down, leading to death.

Treatments: The only way to treat necrotizing fasciitis is to surgically remove any dead or infected tissue. This often necessitates the amputation of one or more limbs. Patients are also given antibiotics to help kill the bacteria. The Centers for Disease Control and Prevention estimates that, even with proper treatment, up to 1 in 5 people with necrotizing fasciitis die.

Recent cases: In 2023, a 58-year-old man in Madrid developed necrotizing fasciitis in both legs, but not at the same time. Doctors initially gave him antibiotics and surgically removed infected and damaged tissue from his right leg. Four days later, they found that the bacteria that caused his infection (S. pyogenes and S. aureus) had also spread to his left leg. Thankfully, after another round of surgery and antibiotics, the man was cured and discharged from the hospital a month later.

Also in 2023, a 52-year-old man in Florida was infected with "flesh-eating" bacteria in his left thigh after being bitten by a relative during a fight at a family gathering. He eventually recovered from the infection, but he had to have 70% of the tissue from the front of his thigh removed and was left with heavy scarring.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!