The 10 Deadliest Viruses

So, what is the deadliest virus on Earth? You’d think this might be an easy enough question to answer but it turns out there is more than one way to define deadliest. For instance is it the virus that kills the most people overall (death rate); or is it the disease that has the highest case fatality rate, i.e. kills the most infected people. For the majority it will be the disease with the highest fatality rate, the one that is a sure-fire death sentence if you ever catch it.

Ironically it is the other diseases, the ones that have reassuringly low case fatality rates that actually kill by the million. There is a reason for this – the viruses which cause the most virulent diseases tend to burn themselves out by killing off their hosts quicker than they can spread. Two particularly good illustrations of this phenomenon are the Ebola virus, with a 90% fatality rate which has killed around 30,000 people to date and the Spanish influenza pandemic which wiped out an estimated 100 million people despite the fact it has a case fatality rate of less than 3%.

As well as the two factors of overall death rate and case fatality rate mentioned above there is the historic dimension – which virus has killed the most people throughout history?

Given these various criteria for defining how deadly a virus is I will be considering all these to provide not only an overall top ten virus list but some separate statistics at the end of the article.

10. Dengue fever

Aedes mosquito

Aedes mosquito | Photo: James Gathany / CDC

Dengue fever is a mosquito-borne infection that was first described nearly 2,000 years ago in China. After gradually becoming established in other countries with populations of  A. aegypti mosquitoes, the range of the disease accelerated in the 18th century. This was attributed to the slave trade and during World War II the spread accelerated further, particularly of the more dangerous forms.
In more recent years globalization has had its impact with dengue rates increasing by 30 times since the 1960s making this a major emerging disease.

Like many of these diseases the vast majority of people have either no symptoms or fairly mild symptoms such as a non-specific fever. Sometimes dengue is referred to as “breakbone fever” which reflects the intense pain which may be felt in the muscles and joints.

For the unlucky few the illness may develop into “severe dengue” with the risk of potentially fatal dengue hemorrhagic fever and dengue shock syndrome. Occurring in less than 5% of cases the main issue here is the increased permeability of the blood vessels. This can result in vomiting blood, organ damage and shock.

Now endemic in 110 countries dengue infects up to 500 million people a year and causes approximately 20,000 deaths. The grim reality is these figures are likely set to rise.

9. Smallpox


Smallpox has been eradicated right? Well, the WHO certified it was no more in 1979, however, both the U.S. and former USSR held samples of the virus for research. There are rumours that after the breakup of the Soviet Union some of these samples were unaccounted for. Even if the smallpox virus was extinct it has been suggested that it could be re-engineered from the digital viral genome and inserted into a related pox virus shell.

The good news is that at present,for all intents and purposes smallpox is extinct in the wild. íHistorically though it has had a devastating effect. Emerging around 10,000 BC there have been a number of epidemics resulting in massive loss of life. Smallpox is both highly contagious and, certainly in historic times, dangerous with fatality rates of up to 90% recorded.

Smallpox was at its most devastating when introduced to the New World by the 18th century European explorers. Whether accidental or not it is estimated that around half of the native aboriginal population of Australia were killed by smallpox during the early years of British colonisation. The disease was similarly disastrous to the native American people and in South America.

Despite the fact that Edward Jenner developed a vaccine for smallpox in 1796 it has been responsible for somewhere in the region of 300 to 500 million deaths since the 1800s alone.

In terms of appearance smallpox is particularly shocking with the body becoming covered in pox – fluid filled pustules. These can occur in the mouth and throat too along with a number of complications including blindness. The mortality rate of the disease largely depends on the course it takes with malignant and hemorrhagic smallpox invariably being fatal.

8. Measles


Child with Measles | © Julien Harneis / License

Most people in the developed world wouldn’t regard measles as being even remotely dangerous. It used to be incredibly common with around 90% of all children having had the measles by the time they were 12. Now-a-days, with routine vaccination in many countries the incidence has been vastly reduced.

What might shock you though is that it is reckoned that between 1855 and 2005 measles was responsible for 200 million deaths worldwide. Even in the 1990s measles was killing over 500,000 people a year. Even today, after the advent of cheap, available vaccines, measles is one of the major causes of death amongst young children with over 100,000 deaths a year.

Where measles has caused the greatest devastation is in communities that have not been previously exposed. In the 16th century Central America was ravaged by measles which was brought over by Europeans. Honduras, for example is said to have lost half its population in a measles epidemic in 1531.

A normal case of measles involves fever, a cough and rash. However, complications are  relatively common and this is where the danger lies. Occurring in around 30% of cases these range from relatively mild e.g. diarrhea through to pneumonia and inflammation of the brain – all of which may be fatal. Other complications include blindness.

7. Yellow fever

Yellow Fever memorial

Yellow Fever memorial – Savannah, Georgia | Photo: © Daniel Mayer / License

Another of history’s big killers is yellow fever. Also known as the “yellow plague” and “vomito negro” (black vomit) ,  this acute haemorrhagic disease has been responsible for a number of serious outbreaks over the centuries.

For most recovery from yellow fever is complete, but in around 15% of cases a second , more serious stage occurs. In these cases there may be bleeding from the mouth, nose, eyes or stomach. Around 50% of the patients who enter this toxic phase are dead within 7 to10 days. Whilst in general a 3% fatality rate is expected during epidemics this has been observed to reach 50%.

As with many of these viral infections it seems the depths of Africa is where it originated. In the early colonial days it was noted that an outbreak in a village would cause the natives to suffer from nothing more serious than flu-like symptoms whereas the majority of European colonists would die. It is thought this difference in severity of the infection was caused by a prolonged, low-level exposure during childhood leading to some degree of immunity.

It could be argued that there was some degree of schadenfreude in the fact that it was slavery and the exploitation of Africa which lead to the epidemics in Europe and North America during the 18th and 19th centuries. Probably the best known of these was the 1792 outbreak in Philadelphia, the then capital of the U.S.. It is said that president George Washington fled the city whilst 10% of those who remained died.
This pattern was mirrored throughout America with an estimated 100,000 to 150,000 deaths during the 18th and 19th centuries.

Today, despite an effective vaccine existing, there are in the region of  an annual 200,000 cases of yellow fever worldwide, killing 30,000 people each year.

6. Lassa virus

Lassa virus

Electron micrograph of Lassa virus | Photo: CDC

You could think of Lassa virus as “Ebola-lite” – but then again it kills as many people in West Africa each year as Ebola did during the height of the 2013-15 epidemic. In addition the symptoms are easily confused with Ebola with both classified as acute viral hemorrhagic fevers. The Lassa virus infects virtually every tissue in the human body and outbreaks are usually started through exposure to the local Mastomys rats.

If you were in any doubt of the dangers posed by Lassa virus then its biosafety level 4 (BSL-4) should convince most of you. This is the highest level of biosafety reserved for working with pathogens that are likely to cause death and for which there are no vaccines or treatments. To put this in perspective, MRSA, HIV and Hepatitis viruses are all classified as biosafety level 2.

Lassa fever causes somewhere in the region of 5,000 deaths annually. Endemic throughout West Africa it is estimated to infect over 300,000 people each year. Whilst a fortunate majority show no symptoms those who do face a fatality rate of 15-20%. In epidemics the fatality rate from Lassa virus has been seen to reach 50%. Not quite that of Ebola or Marburg viruses, but a scary number all the same.