The 12 Deadliest Virus Outbreaks

Throughout history, humans have fought many deadly viruses. Some viral diseases have declined through vaccination, others through antiviral drugs. We have even succeeded in eradicating smallpox completely.

However, other viruses can emerge, requiring new solutions. Some viruses, including the novel coronavirus currently causing a worldwide outbreak, have relatively low mortality rates. Nevertheless, because we have no tools to combat them, they pose a major public health threat.

In recent years, several viruses have jumped from animals to humans and triggered major outbreaks. The virus responsible for the 2014-2016 Ebola outbreak in West Africa is the deadliest member of the Ebola family, killing 90% of those it infects.

Here are the 12 deadliest viruses humanity has encountered:

Marburg Virus

In 1967, scientists noticed a small outbreak among laboratory workers in Germany exposed to infected monkeys brought from Uganda. Marburg virus was responsible. It resembles Ebola because both can cause hemorrhagic fever: high fever and bleeding that can lead to shock, organ failure and death.

According to the World Health Organization (WHO), mortality in the first outbreak was 25%. However, it exceeded 80% during outbreaks in the Democratic Republic of the Congo in 1998-2000 and Angola in 2005.

Ebola Virus

The first known human Ebola outbreaks occurred simultaneously in the Republic of the Sudan and the Democratic Republic of the Congo in 1976. Ebola spreads through contact with blood, other body fluids or tissues from infected people or animals. Mortality varies between known subtypes.

According to the WHO, the West African outbreak began in early 2014 and remains the largest and most complex outbreak of the disease to date.

Rabies

Rabies, caused by the rabies virus, has become very rare in developed societies thanks to domestic animal vaccines introduced in the 1920s. It remains a serious problem in India and parts of Africa. Vaccination can provide complete treatment against this deadly disease affecting the brain; without vaccination, its fatality rate is 100%.

HIV

HIV may still be the modern world’s greatest killer. Since its identification in the early 1980s, it has caused 32 million deaths. HIV is also the most costly disease.

Powerful antiviral drugs have made it possible for people to live with HIV for years. However, it continues to burden many low- and middle-income countries, where 95% of new HIV infections occur. According to the World Health Organization, one in every 25 adults in the African region is HIV-positive, accounting for more than two-thirds of people living with HIV worldwide.

Smallpox

In 1980, the World Health Organization declared the world free of smallpox. Before that, humans had fought it for thousands of years, and it killed around a third of those infected. Survivors were left with permanent scars and often blindness.

Historians estimate that 90% of the Indigenous population of the Americas died from smallpox carried by European explorers. In the 20th century alone, smallpox caused 300 million deaths.

Hantavirus

Hantavirus was first identified in the USA in 1993 after a healthy young man and his fiancée developed shortness of breath. Their deaths within days attracted considerable attention. A few months later, health officials isolated the virus from a mouse living in the home of an infected person. According to the Centers for Disease Control and Prevention (CDC), more than 600 people in the USA had been infected with hantavirus, and 36% had died.

Hantavirus does not spread from one person to another. People become infected through exposure to the droppings of infected mice.

According to a 2010 article in Clinical Microbiology Reviews, another hantavirus caused an outbreak during the Korean War in the early 1950s. More than 3,000 troops were infected, and around 12% of those who became ill died.

Flu (Influenza)

According to the World Health Organization, a typical flu season can cause around 500,000 deaths worldwide. However, a new flu strain sometimes emerges, causing pandemics with faster spread and higher mortality.

- Related Topic: What Are Epidemics and Pandemics? How Do They Differ?

The deadliest flu outbreak, known as Spanish flu, is one example. Beginning in 1918, it is thought to have infected around 40% of the world’s population and caused 50 million deaths.

Dengue Fever

Dengue virus first appeared in the Philippines and Thailand in the 1950s. From there, it spread to tropical and subtropical regions worldwide. Around 40% of the world’s population lives where dengue occurs. As the world warms, the disease and its mosquito vectors are spreading further.

According to the World Health Organization, dengue fever infects 50 to 100 million people annually. Although its fatality rate is lower than that of other viruses, 2.5% of patients develop an Ebola-like illness called dengue hemorrhagic fever. Untreated, this condition has a mortality rate of up to 20%.

Rotavirus

Two vaccines are available to protect against rotavirus, which causes severe diarrhea in infants and young children. The virus can spread rapidly through the fecal-oral route: tiny fecal particles contaminate food or drinking water, which people then consume.

Although children in developed countries rarely die from rotavirus, it can be fatal in developing countries where fluid-replacement treatment is not widely available.

The World Health Organization estimates that 453,000 children younger than 5 died from rotavirus infection worldwide in 2008. However, countries introducing vaccination have reported sharp declines in rotavirus-related hospitalizations and deaths.

SARS-CoV-1

According to the World Health Organization, the virus that causes severe acute respiratory syndrome (SARS) first appeared in Guangdong province, southern China, in 2002. It probably originated in bats and passed to nocturnal mammals called civets before infecting humans. After causing an outbreak in China, SARS spread to 26 countries, infected more than 8000 people and caused over 770 deaths in two years.

The disease causes fever, chills and body aches, often progressing to severe pneumonia in which the lungs become inflamed. Its estimated fatality rate is 9.6%, and no treatment or vaccine has yet been approved. According to the CDC, however, no new SARS cases have been reported since the early 2000s.

SARS-CoV-2 (Novel Coronavirus, COVID-19 Pandemic)

SARS-CoV-2 belongs to the same large virus family as SARS-CoV, known as coronaviruses, and was first identified in Wuhan, China, in December 2019. Like SARS-CoV, it probably originated in bats and is thought to have passed through an intermediate animal host before infecting humans.

Since the first case, the virus has infected tens of thousands of people in China and thousands worldwide. The ongoing outbreak has led to extensive quarantines in Wuhan and nearby cities, travel restrictions in affected countries, and worldwide efforts to develop diagnostics, treatments and vaccines.

The disease caused by SARS-CoV-2, called COVID-19, has an estimated mortality rate of around 2.3%. Evidence suggests it is more severe in older people or those with other medical conditions. Common symptoms include fever, dry cough and shortness of breath. Severe cases may progress to pneumonia.

MERS-CoV

The virus causing Middle East respiratory syndrome (MERS) led to outbreaks in Saudi Arabia in 2012 and South Korea in 2015. It belongs to the same family as SARS-CoV and SARS-CoV-2 and probably originated in bats. It is known to have infected camels before passing to humans, in whom it causes fever, cough and shortness of breath.

MERS often progresses to severe pneumonia and has an estimated mortality rate of 30% to 40%, making it the deadliest coronavirus capable of jumping from animals to humans. Like SARS-CoV and SARS-CoV-2, MERS has no approved treatment or vaccine.

What About Plague?

The organism responsible for plague, Yersinia pestis, is a bacterium, so it falls outside this article’s scope. However, because it caused history’s deadliest pandemics, let us briefly discuss it:

Plague has been known since antiquity and is nicknamed the "Black Death." In the Middle Ages, between 1347-1353, it caused the loss of a third of Europe’s population. Between 1347-1348, it also reduced Venice’s population from 130,000 to 70,000. It can be treated easily with modern antibiotics. Although eliminated from all developed and many developing countries, it still occurs in parts of Asia and Africa.

It is contagious and deadly. Although commonly believed to spread from rats, the bacteria are actually transmitted by a type of flea; rats are also victims of the disease.

The video below shows historical epidemics and the numbers of people who died in them.

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