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The deadliest viruses in history

3D illustration of influenza, one of the world's deadliest viruses.
(Image credit: Shutterstock)

Humans have been battling viruses since before our species had even evolved into its modern form. For some viral diseases, vaccines and antiviral drugs have allowed us to keep infections from spreading widely, and have helped sick people recover. For one disease — smallpox — we've been able to eradicate it, ridding the world of new cases.

But we're a long way from winning the fight against viruses. In recent decades, several viruses have jumped from animals to humans and triggered sizable outbreaks, claiming thousands of lives. The viral strain that drove the 2014-2016 Ebola outbreak in West Africa kills up to 90% of the people it infects, making it the most lethal member of the Ebola family.

Related: 20 of the worst epidemics and pandemics in history

But there are other viruses out there that are equally deadly, and some that are even deadlier. Some viruses, including the novel coronavirus currently driving outbreaks around the globe, have lower fatality rates, but still pose a serious threat to public health as we don't yet have the means to combat them. 

Here are the 12 worst killers, based on the likelihood that a person will die if they are infected with one of them, the sheer numbers of people they have killed, and whether they represent a growing threat.

Marburg virus

Marburg virus

The Marburg virus causes hemorrhagic fever in humans and non-human primates. (Image credit: ROGER HARRIS/SCIENCE PHOTO LIBRARY via Getty Images)

Scientists first identified the Marburg virus in 1967, when small outbreaks occurred among lab workers in Germany who were exposed to infected monkeys imported from Uganda, according to the World Health Organization (WHO). Marburg virus symptoms are similar to Ebola in that both viruses can cause hemorrhagic fever, meaning that infected people develop high fevers, and bleeding throughout the body that can lead to shock, organ failure and death, according to Mayo Clinic.

The case fatality rate in the first outbreak (1967) was 24%, but it was 83% in the 1998-2000 outbreak in the Democratic Republic of Congo, and 100% in the 2017 outbreak in Uganda, according to the WHO

The first known Marburg virus outbreak in West Africa was confirmed in August 2021. The case was a male from south-western Guinea, who developed a fever, headache, fatigue, abdominal pain and gingival hemorrhage. This outbreak lasted for six weeks and, while there were 170 high-risk contacts, only one case was confirmed, according to Reuters.

Ebola virus

Ebola virus

Microscopic image of an Ebola virus (Image credit: Shutterstock)

The first known Ebola outbreaks in humans struck simultaneously in the Republic of the Sudan and the Democratic Republic of Congo in 1976. Ebola is spread through contact with blood or other body fluids, or tissue from infected people or animals. The known strains vary dramatically in their deadliness, Elke Muhlberger, an Ebola virus expert and associate professor of microbiology at Boston University, told Live Science.

One strain, Ebola Reston, doesn't even make people sick, according to Essential Human Virology (2016). But for the Bundibugyo strain, the fatality rate is up to 50%, and it is up to 71% for the Sudan strain.

The outbreak underway in West Africa began in early 2014, and is the largest and most complex outbreak of the disease to date, according to WHO

In December 2020, the Ervebo vaccine was approved by the US Food and Drug administration. This vaccine helps to defend against the Zaire ebola virus and a global stockpile became available from January 2021.

Rabies

Rabies virus

This image of the rabies virus, taken through an electron microscope, shows particles of the virus itself, as well as the round structures called Negri bodies, which contain viral proteins. (Image credit: CDC/ Dr. Fred Murphy)

Although rabies vaccines for pets, which were introduced in the 1920s, have helped make the disease exceedingly rare in the developed world, this condition remains a serious problem in India and parts of Africa.

Infection from this virus develops after a bite or scratch from an infected animal. This can result in damage to the brain and nerves. Once symptoms begin to show, death almost always follows, according to the National Health Service (NHS).

"It destroys the brain, it's a really, really bad disease," Muhlberger said. "We have a vaccine against rabies, and we have antibodies that work against rabies, so if someone gets bitten by a rabid animal we can treat this person," she said.

However, she said, "if you don't get treatment, there's a 100% possibility you will die."

HIV

HIV SEM image

This scanning electron microscope image shows the human immunodeficiency virus (HIV, in green), infecting a cell.  (Image credit: Cynthia Goldsmith, Centers for Disease Control and Prevention)

In the modern world, the deadliest virus of all may be HIV. "It is still the one that is the biggest killer," said Dr. Amesh Adalja, an infectious disease physician and spokesman for the Infectious Disease Society of America.

An estimated 32 million people have died from HIV since the disease was first recognized in the early 1980s. "The infectious disease that takes the biggest toll on mankind right now is HIV," Adalja said.

Powerful antiviral drugs have made it possible for people to live for years with HIV. But the disease continues to devastate many low- and middle-income countries, where 95% of new HIV infections occur. 

Nearly 1 in every 25 adults within the WHO African region is HIV-positive, meaning that there are over two-thirds of the people living with HIV worldwide, according to the WHO. In 2020, there were 680,000 HIV-related deaths worldwide. 

Smallpox

A smallpox virus

A smallpox virus (Image credit: CDC/ J. Nakano)

In 1980, the World Health Assembly declared the world free of smallpox. But before that, humans battled smallpox for thousands of years, and the disease killed about 1 in 3 of those it infected, according to the BBC. It left survivors with deep, permanent scars and, often, blindness.

Mortality rates were far higher in populations outside of Europe, where people had little contact with the virus before visitors brought it to their regions.  For example, historians estimate 90% of the native population of the Americas died from smallpox introduced by European explorers. In the 20th century alone, smallpox killed 300 million people, the BBC reported.

"It was something that had a huge burden on the planet, not just death but also blindness, and that's what spurred the campaign to eradicate from the Earth," Adalja said.

Hantavirus

Sin Nombre virus

This image shows the hantavirus known as the Sin Nombre virus (SNV), under a transmission electron microscope. This virus caused an outbreak in November 1993, in the Four Corners region of the U.S.  (Image credit: Cynthia Goldsmith. Provided by CDC/ Brian W.J. Mahy, PhD; Luanne H. Elliott, M.S.)

Hantavirus pulmonary syndrome (HPS) first gained wide attention in the U.S. in 1993, according to the Centers for Disease Control and Prevention (CDC). A healthy, young Navajo man and his fiancée living in the Four Corners area of the United States died within days of developing shortness of breath. A few months later, health authorities isolated hantavirus from a deer mouse living in the home of one of the infected people. More than 600 people in the U.S. have now contracted HPS, and 36% have died from the disease, according to the CDC.

The virus is not transmitted from one person to another, rather, people contract the disease from exposure to the droppings of infected mice.

Previously, a different hantavirus caused an outbreak in the early 1950s, during the Korean War, according to a 2010 paper in the journal Clinical Microbiology Reviews. More than 3,000 United Nations troops became infected, and about 12% of them died.

While the virus was new to Western medicine when it was discovered in the U.S., researchers realized later that Navajo medical traditions describe a similar illness, and linked the disease to mice.

Influenza

Influenza virus

This digitally-colorized image shows the H1N1 influenza virus under a transmission electron microscope. In 2009, this virus (then called the swine flu) caused a pandemic, and is thought to have killed 200,00 people worldwide. (Image credit: National Institute of Allergies and Infectious Diseases (NIAID))

During a typical flu season, up to 650,000 people worldwide will die from the illness, according to WHO. But occasionally, when a new flu strain emerges, a pandemic results with a faster spread of disease and, often, higher mortality rates.

The most deadly flu pandemic, sometimes called the Spanish flu, began in 1918 and sickened up to 40% of the world's population, killing an estimated 50 million people, according to CDC.

"I think that it is possible that something like the 1918 flu outbreak could occur again," Muhlberger said. "If a new influenza strain found its way in the human population, and could be transmitted easily between humans, and caused severe illness, we would have a big problem."

Dengue

Dengue virus

This image shows round, Dengue virus particles as they look under a transmission electron microscope. Dengue viruses are transmitted to humans by the bite of an infected mosquito. (Image credit: Frederick Murphy. Provided by CDC/ Frederick Murphy, Cynthia Goldsmith)

Dengue virus first appeared in the 1950s in the Philippines and Thailand, and has since spread throughout the tropical and subtropical regions of the globe, according to Clinical Microbiology Reviews. Up to 40% of the world's population now lives in areas where dengue is endemic and the disease, according to the journal Nature — with the mosquitoes that carry it — is likely to spread farther as the world warms.

Dengue sickens 100 to 400 million people a year, according to WHO. Although the mortality rate for dengue fever is lower than some other viruses, at around 1%, the virus can cause an Ebola-like disease called dengue hemorrhagic fever, and that condition has a mortality rate of 20% if left untreated. "We really need to think more about dengue virus because it is a real threat to us," Muhlberger said. 

A vaccine for Dengue was approved in 2019 by the U.S. Food and Drug Administration for use in children 9-16 years old living in an areas where dengue is common and with a confirmed history of virus infection, according to the CDC. In some countries, an approved vaccine is available for those 9-45 years old, but again, recipients must have contracted a confirmed case of dengue in the past. Those who have not caught the virus before could be put at risk of developing severe dengue if given the vaccine.  

Rotavirus

Rotavirus

Rotaviruses particles are shown here under a very high magnification of 455,882X. (Image credit: CDC/ Dr. Erskine L. Palmer)

Two vaccines are now available to protect children from rotavirus, the leading cause of severe diarrheal illness among babies and young children. The virus can spread rapidly, through what researchers call the fecal-oral route (meaning that small particles of feces end up being consumed).

Although children in the developed world rarely die from rotavirus infection, the disease is a killer in the developing world, where rehydration treatments are not widely available.

The WHO estimates that worldwide, there are more than 25 million outpatient visits and two million hospitalizations each year due to rotavirus infections. Countries that have introduced the vaccine have reported sharp declines in rotavirus hospitalizations and deaths.

SARS-CoV

SARS-Cov

(Image credit: CDC/ Dr. Fred Murphy)

The virus that causes severe acute respiratory syndrome, or SARS, was first identified in 2003 during an outbreak in China, according to the WHO. The virus likely emerged in bats initially, then hopped into nocturnal mammals called civets before finally infecting humans, according to the Journal of Virology. After triggering an outbreak in China, SARS spread to 26 countries around the world, infecting more than 8000 people and killing more than 770 over the course of several months, according to History.com

The disease causes fever, chills and body aches, and often progresses to pneumonia, a severe condition in which the lungs become inflamed and fill with pus. SARS has an estimated mortality rate of 9.6%, however, no new cases of SARS have been reported since the early 2000s, according to the CDC

SARS-CoV-2

SARS-CoV-2

This transmission electron microscope image shows SARS-CoV-2– the virus that causes COVID-19– isolated from a patient in the US.  (Image credit: NIAID-RML)

SARS-CoV-2 belongs to the same large family of viruses as SARS-CoV, known as coronaviruses, and was first identified in December 2019 in the Chinese city of Wuhan. The virus may have originated in bats and passed through an intermediate animal before infecting people, according to Nature

The initial outbreak prompted an extensive quarantine of Wuhan and nearby cities, restrictions on travel to and from affected countries and a worldwide effort to develop diagnostics, treatments and vaccines. Since its appearance, the virus has caused over four million deaths worldwide.

The disease caused by SARS-CoV-2, called COVID-19, poses a higher risk to people who have underlying health conditions, according to WHO. Common symptoms include fever, cough, loss of taste or smell and shortness of breath and more serious symptoms include breathing difficulties, chest pain and loss of mobility.

On August 23, 2021 the U.S. Food and Drug administration approved the first COVID-19 vaccine, called Pfizer-BioNTech. In December 2020, this vaccine became the first to be approved after a large clinical trial, according to Nature

MERS-CoV

an illustration of the MERS virus, a type of coronavirus

(Image credit: Shutterstock)

The virus that causes Middle East respiratory syndrome, or MERS, sparked an outbreak in Saudi Arabia in 2012 and another in South Korea in 2015. The MERS virus belongs to the same family of viruses as SARS-CoV and SARS-CoV-2. According to WHO, the disease infected camels before passing into humans and can trigger fever, coughing and shortness of breath in infected people. 

MERS, which is most common in the Middle East, often progresses to severe pneumonia and has an estimated mortality rate of around 35%. There is no vaccine available to prevent this disease, according to the NHS. The best way to reduce the chances of infection is to wash hands regularly, avoid contact with camels and not consume products containing raw animal milk. 

Follow us @livescience, Facebook & Google+. Original article on Live Science.

This article was updated on Oct. 15, 2021 by How It Works staff writer Ailsa Harvey.

  • Don't
    Not sure where the name SARS-CoV-2 came from. The official WHO name is COVID-19.
    Reply
  • Meh
    Don't said:
    Not sure where the name SARS-CoV-2 came from. The official WHO name is COVID-19.

    The name of the virus itself, free floating without having infected anyone is SARS-CoV-2 (there's also two strains of it, an S and and L strain). When someone is infected with SARS-CoV-2, the disease is called COVID-19. One is the name of the infectious particle, the other is the name of the disease.
    Reply
  • Don't
    Meh said:
    The name of the virus itself, free floating without having infected anyone is SARS-CoV-2 (there's also two strains of it, an S and and L strain). When someone is infected with SARS-CoV-2, the disease is called COVID-19. One is the name of the infectious particle, the other is the name of the disease.
    Sorry - my mistake. You are correct. COVID-19 is the name of the illness and not the virus itself, and the article is about the viruses. Had I read more closely, my comment would have been "Not sure where the name SARS-CoV-2 came from. The official WHO name is 2019-nCoV ." I am new to all this. My background is engineering - not infectious organisms or diseases. Now having dug a little deeper, I see different groups seem to have different names for the same virus. I note that the WHO says COVID-19 is different from SARS. So that has me wondering why anyone would decide to use SARS in the name if a virus that does not (according to WHO at least) cause SARS.
    Reply
  • Jim Thompson MD
    Don't said:
    Sorry - my mistake. You are correct. COVID-19 is the name of the illness and not the virus itself, and the article is about the viruses. Had I read more closely, my comment would have been "Not sure where the name SARS-CoV-2 came from. The official WHO name is 2019-nCoV ." I am new to all this. My background is engineering - not infectious organisms or diseases. Now having dug a little deeper, I see different groups seem to have different names for the same virus. I note that the WHO says COVID-19 is different from SARS. So that has me wondering why anyone would decide to use SARS in the name if a virus that does not (according to WHO at least) cause SARS.
    See here: https://www.nature.com/articles/s41564-020-0695-z" Thus, the reference to SARS in all these virus names (combined with the use of specific prefixes, suffixes and/or genome sequence IDs in public databases) acknowledges the phylogenetic (rather than clinical disease-based) grouping of the respective virus with the prototypic virus in that species (SARS-CoV). The CSG chose the name SARS-CoV-2 based on the established practice for naming viruses in this species and the relatively distant relationship of this virus to the prototype SARS-CoV in a species tree and the distance space"
    Reply
  • drpaburke
    Comment on rabies fatality rate. The classic exposure /infection/terminology is difficult to employ when speaking about rabies. Although all warm-blooded animals are thought to be susceptible to rabies, there are strains of the rabies virus ( multiple bat stains ) strains are maintained in particular reservoir host(s), with some cross over especially in the US between raccoons and skunks. Although a strain can cause rabies in other species, the virus usually dies out during serial passage in species to which it is not adapted, and non-carnivores (cows, horses, deer, groundhogs, beavers ) AND CATS, like small rodents, are dead-end hosts. The CDC estimates in the US, 1 million dollars per potential life saved is spent by post-exposure prophylaxis in cases of exposure to animals other than bats, canines, fox, raccoon, skunks. At some point, the inability to PROVE this may be trumped by statistics; when I became a veterinarian in 1975, PEP was still recommended for squirrel and gerbil bites. Hundreds of (unvaccinated) cats are infected with, and die ( or are euthanized) of rabies each year -no way every human exposure to "the kitten in the park " is tracked down. Certainly, many farmers and ranchers are unknowingly exposed. Yet almost all of the 6-9 people diagnosed in the US yearly, knew they were bitten by a dog (when outside the US) or handled a bat. And there have been several incidences since 2000, where people got rabies secondary to solid organ transplants. This had been thought only a risk when transplanting 'nerve' tissue (corneas), I wonder if this reflects better and/or different immunosuppressive drugs in recipients.

    Species vary in susceptibility to various strains, humans are 'most' susceptible to canine rabies and, in the US, the silver-haired bat strain. This is a solitary bat with infrequent human interaction, whereas we have much more exposure to big and little brown bats and Mexican free-tailed bats. (Only a small percentage of any of these have rabies, -it kills them too!)

    The virus needs to get to a nerve, so if a bite is not deep enough, or a small viral load is deposited, or the 'victim' immune system responds - an infection will never be established. If the virus is able to get to a nerve, it attempts to travel up an axon, to the brain- again, the immune system may eliminate. As rabies is a slow virus, it can self -immunize, explaining the presence of rabies neutralizing antibodies in Amazonian Indians and others who have never been vaccinated? (The reason why a mature dog is considered immunized 28 days after its first rabies vaccination, is if it has been exposed or is 'incubating' rabies virus but the virus is more than 28 days away the vaccine will prevent infection. Although antiglobulin is given, PEP - a killed vaccine, is basically, rapid immunization. I am pulling this from memory but I believe in cases where multiple people were bitten by a rabid dog, in 15% infection was established. Once the the virus is in the brain and /or clinical signs are seen, then it is almost always, fatal. Since definitive diagnosis is made on brain biopsy, the apparent spontaneous cures or response to treatment remain unproven.

    Not to diminish the threat or the misery of this disease. I don't understand why the WHO estimate of deaths has been quoted as 35,000 to 55,000 for the last 40 years - while the world population went from approximately 4 billion to 8 billion, mostly in Africa and Asia where few dogs are vaccinated and most cases are seen.
    Reply
  • nemoyap
    admin said:
    Humans have been fighting viruses throughout history. Here are the 12 viruses that are the world's worst killers, based on their mortality rates, or the sheer numbers of people they have killed.

    ... on COVID-19 origin

    "The virus likely originated in bats, like SARS-CoV, and passed through an intermediate animal before infecting people. "
    ...
    dear admin, i don't see solid evidence and facts that this new strain of virus originated from Bats, passing thru animals, then to human. Should we sample and study the Bats or animal carcasses for more evidence? Even the latest chinese scientists from Wuhan flip to suggest that other sources is potential. Indonesia market for example has high appetite for Bats as food.

    It seems that humans are passing the virus to dogs and cats now. also, it is well known there is a P4 virus lab located nearby the Wuhan market. my question - is there any regulations that governs the building of such dangerous lab so close to the mass human neighbourhood?
    Reply
  • SpaceTeddy
    Jim Thompson MD said:
    See here: https://www.nature.com/articles/s41564-020-0695-z" Thus, the reference to SARS in all these virus names (combined with the use of specific prefixes, suffixes and/or genome sequence IDs in public databases) acknowledges the phylogenetic (rather than clinical disease-based) grouping of the respective virus with the prototypic virus in that species (SARS-CoV). The CSG chose the name SARS-CoV-2 based on the established practice for naming viruses in this species and the relatively distant relationship of this virus to the prototype SARS-CoV in a species tree and the distance space"

    And in any case, the World Health Organization (WHO) is not the naming authority for novel viruses — this is the job of the International Committee on Taxonomy of Viruses (ICTV) and in this case specifically the Coronaviridae Study Group (CSG, or ICTV-CSG) which concluded that the virus that causes COVID-19 should be named SARS-CoV-2

    The WHO on the other hand is the naming authority for novel diseases, and the name 2019-nCoV for the virus causing COVID-19 was only of a provisional nature, signifying a novel coronavirus discovered in 2019. Official classification of viruses is a scientific process, where the degree of relatedness (of novel viruses to those previously identified) is considered. By actually comparing the genome of the novel coronavirus to the genomes of related viruses, looking at certain replicative proteins, it was clear that SARS-CoV (causing SARS) and SARS-CoV-2 (causing COVID-19) are quite close to each other genetically, even though nothing indicates that the latter is a direct descendent of the former. Both are also much more closely related to other coronaviruses, known to infect Asian and African bats respectively. In contrast, none of them are as closely related to MERS-CoV as they are to each other. On the other hand, these three are more closely related to each other, than any of them are to the other coronaviruses known to infect humans. The three previously discussed (causing major epidemics in recent decades) are zoonotic viruses, meaning they are believed to momentarily “spill over” from animals to humans. The other four coronaviruses infecting humans are common respiratory viruses that circulate continually among us, with symptoms ranging from the common cold (which can be caused by more than 200 virus strains) to more high-morbidity outcomes
    Reply
  • victorlater
    it tells you that first was sars 1 that came from bats and hopped into a nocturnal mammal called videts then sars 2 or covid 19 which also came from bats and possibly hopped into other mammals like The Pangolin that has a 99% identical match to the virus. then the sars 3 they al are a family of Coronaviruses.
    Reply
  • seagrape
    Don't said:
    Not sure where the name SARS-CoV-2 came from. The official WHO name is COVID-19.
    The name given to the virus is SARS-Cov-2. The name given to the disease caused by the virus is COVID-19.
    Reply
  • seagrape
    The name given to the virus is SARS-Cov-2. The name given to the disease caused by the virus is COVID-19.
    Reply