For doctors, trying to figure out whether a person has a parasitic infection is like solving a mystery with few clues or only vague ones.
In the following eight cases, parasites wreaked havoc inside a person's body before being detected. Some of these nasty organisms were picked up by people in strange ways, while others wound up in unusual locations.
All of them caused trouble until their mysteries were solved.
Roundworms camp out in a woman's stomach.
A Japanese woman who liked eating raw fish ended up in the hospital after eating raw salmon that was contaminated with parasitic worms, according to a 2016 case in The New England Journal of Medicine.
The roundworms hitched a ride on the uncooked fish and eventually tunneled their way into the walls of her stomach. Within several hours of the meal, the 36-year-old developed stomach and chest pains, along with nausea and vomiting.
But she didn't go to the hospital until two days later. There, doctors inserted a small camera into her stomach and noticed something fishy: 11 anisakis larvae, which are a type of parasitic roundworm. Anisakis worm infections are more common in countries where people eat raw fish or undercooked seafood. But typically, when these parasitic infections happen, one or two worms are usually to blame — not 11 of them, her doctor told Live Science in 2016.
After the worms were fished out of the woman's stomach, her symptoms went away.
Fungal spores infect transplanted organs.
Three people who received transplanted organs from the same donor developed brain problems shortly after the operations due to a rare parasitic infection that originated in the organ donor's body.
One man, who received a kidney, experienced behavior changes. He later died, and his doctors wondered whether the transplanted organ might have been responsible.
Indeed, the man's autopsy revealed his tissue tested positive for a tiny parasite known as Encephalitozoon cuniculi, which belongs to a group of single-celled organisms called Microsporidia.
Doctors tracked down the person who received the donor's liver, and this individual had tremors and difficulty walking. A third person, who received the donor's heart and her other kidney, had been hospitalized with a type of brain inflammation known as encephalitis.
Test results showed that both recipients were infected with Microsporidia. They took medication to treat the parasite and no longer had neurological complaints.
The case series was presented in 2015 at IDWeek, a meeting of several organizations focused on infectious diseases.
Teensy parasite grows on contact lens.
A young woman in Florida developed a serious infection in her left eye from a parasite that grew on her contact lens, a TV news report from 2013 revealed.
The parasite, called Acanthamoeba, is found in water and soil worldwide, and can enter a person's body through cuts on the skin, contact-lens solution or inhalation, according to the Centers for Disease Control and Prevention (CDC). Anyone who wears contact lenses can become infected with the parasite, though it is incredibly rare, affecting only about 1 in 33 million contact-lens wearers, the CDC estimates.
If the teenager had not sought help for the pain, swelling and redness in her left eye, the infection may have led to permanent vision problems or blindness. She was treated for several months with medication to clear the infection, and said she would be more diligent about cleaning her contacts and not leaving them in her eyes too long.
Beef tapeworm lives in man's small intestine for two years.
A man in China got a 20-foot-long tapeworm by eating raw beef, a 2016 case report in The New England Journal of Medicine revealed.
Based on the size of the tapeworm, his doctors suspected the parasite had attached itself to the man's small intestine and lived there for more than two years. But the infection may have gone undetected because the parasite doesn't always cause immediate symptoms or may cause mild ones, such as an upset stomach.
The 38-year-old man was diagnosed when he went to his doctor's office complaining of stomach pain, vomiting and a 20-lb. (9 kilograms) weight loss in three days. He also brought with him a fragment that he had found in his poop, which eventually was identified as the species Taenia saginata.
The man took a medication that caused him to expel the long tapeworm from his body, and several months later, his appetite and weight returned to normal.
Hookworm creeps into woman's knee.
A woman returned from her Caribbean vacation with more than just a great tan: She also came home with a nasty parasite in her skin, according to a 2017 report of her case published in the Journal of Emergency Medicine.
Two weeks after the woman's trip, her knee developed an itchy rash that left a snake-like pattern on her skin. The rash turned out to be a "textbook example" of an infection called cutaneous larva migrans, a hookworm infection in which a trail is left behind as the worm burrows through the skin, her doctor told Live Science.
Hookworms are a common parasitic infection, typically found in warm, moist environments. But in this woman's case, her infection was caused by a worm that usually doesn't bother people, and is more likely to infect dogs and cats instead.
It's not clear how she picked up the hookworm, but because humans are not the normal hosts for this parasite, the worms can't reproduce and complete their life cycle in the human body. So, after the worm enters the skin and creeps around for a while, it eventually dies.
A person's immune system can attack the hookworm, which causes intense itching and leaves behind the curvy, inflamed rash found on the woman's knee known as a "creeping eruption." She was given an anti-parasitic drug to treat the infection.
Pork tapeworm curls up in a man's small intestine.
Doctors in India removed a 6-foot-long (1.8 meters) pork tapeworm through a man's mouth — while he was sedated, of course — after they spotted the lengthy parasite curled up in his small intestine, according to a 2017 report of the man's case published in The New England Journal of Medicine.
Doctors discovered the worm nestled in the man's small intestine when they inserted a small camera into the man's mouth and down to the stomach and upper part of the small intestine.
The doctors had also performed a colonoscopy to look at the man's large intestine. There, they found a smaller segment of the worm.
People can get pork tapeworms by eating raw or undercooked pork, and the worms can live in a person's gut for a while before causing any symptoms.
After removing the lanky tapeworm through the man's mouth, doctors treated him with anti-parasitic medication to kill any remnants of the worm.
Tapeworm inside a Colombian man develops cancer.
The tumors found on a Colombian man's lung came from an unlikely source. They were not made up of human cells. Rather, they were composed of cancer cells from a tapeworm living inside of him, according to a 2015 report of the man's case published in The New England Journal of Medicine. In other words, the worm got cancer inside of the man.
The authors of the report describing this extremely rare case suspect that the 41-year-old man, who had HIV and was not taking his medications for the virus, probably had a weak immune system. As a result, his body couldn't fight the tapeworm infection.
As the parasite grew inside the man, the tapeworm got cancer, and those cancer cells spread to the man, causing tumors that formed in his lungs. The doctors who treated the man sent the tumor cells to the Centers for Disease Control and Prevention, where health officials identified the parasite as a tapeworm called Hymenolepis nana, which is the most common tapeworm found in humans.
However, the man died just three days after learning his tumors were caused by the parasite.
A "fluke" worms its way into a teen's eye.
Doctors determined that the reason a teen boy in Mexico could barely see out of his right eye was because a parasitic flatworm was "moving freely" in his eyeball, according to a September 2017 case report in The New England Journal of Medicine.
An eye exam revealed that the worm had burrowed holes in the boy's eyeball and was wriggling in and out of it. And his cornea (the translucent layer of the eye that covers the iris and pupil) was dotted with blood.
Doctors needed to surgically remove the worm, which was identified as a trematode or "fluke." During the operation, doctors noticed that the parasite had also badly damaged the retina, the tissue lining the back of the eye.
Parasitic-worm infections in the eye are extremely rare, the teen's doctor told Live Science in September. Six months after the fluke was removed, the teen still had vision problems in his eye.