The 10 Deadliest Viruses

5. Hepatitis

Hepatitis C virus

Hepatitis C virus | Photo; ©Blausen Medical Communications / License

Hepatitis is the name given to collection of viral diseases which attack the liver. There are 5 forms of contagious hepatitis which are given letters A to E. Of all these the most serious are hepatitis B and hepatitis C which together cause almost a million deaths per annum. These are often passed from mother to child but can also be spread via blood transfusions, tattooing, dirty syringes and sexual activity.

Hepatitis B causes the largest share of fatalities each year (around 700,000). It is a fairly unremarkable illness with no gory symptoms. The majority of deaths are the result of the illness slowly attacking the liver over a period of years eventually resulting in either liver cancer or cirrhosis. Whilst becoming infected with hepatitis B as an adult usually results in an acute episode of illness which ends up with a full recovery, those infected as children are more likely to develop the long term condition.

Although the overall death rate from hepatitis C is lower than B it still kills an estimated 350,000 people a year, mostly in the developing world. Figures suggest that there are around 200 million people (or 3% of the global population) living with hepatitis C.

4. Rabies


Patient in last stages of Rabies / Photo: CDC

Rabies is one of several deadly diseases belonging to the Lyssavirus genus. This name is derived from Lyssa, the Greek goddess of rage, madness and frenzy, and the word rabies itself comes from the latin for “madness”. It has been one of mankind’s most feared diseases since ancient times and with good reason.

The best known form of rabies is referred to as “furious rabies” and affects 80% of those infected. This stage includes the classic symptoms of confusion, agitation, paranoia and terror. An infected person may also display hydrophobia (fear of water). In this seemingly bizarre condition the patient becomes panicked when given anything to drink. The most likely reason is that rabies infects the salivary glands at the back of the mouth so it can be passed on in a bite. This infection also causes the muscles of the throat to go into excruciating spasm when salivation increases on presentation of a drink.

Infection with rabies occurs when bitten or scratched by an infected animal – most commonly dogs or bats. Whilst there may be some flu-like symptoms after the bite the disease is generally without symptoms during an incubation phase. This usually lasts for 1 to 3 months but can take years as the infection creeps through the nervous system towards the brain.

Rabies is difficult to diagnose and unless suspected after a bite may well go undetected until neurological symptoms develop. At this point it is almost definitely too late for the patient; rabies has an almost 100% fatality rate within a matter of days . In fact only 5 people have ever survived rabies and the first was Jeanna Giese in 2005. She was treated with a new approach (the Milwaukee protocol) involving an induced coma and survived to make an almost full recovery. Despite the success in this case, this method still only has an 8% chance of success.

Fortunately a bite from a rabies infected animal is no longer a certain death sentence. If you can receive medical treatment in the form of post-exposure prophylaxis (PEP) within 10 days there is an almost 100% chance you will survive. There is also an equally effective vaccine.
Nevertheless, nearly 60,000 people die of rabies every year, mostly in Africa and southern Asia. Over a third of these deaths are in India and dog bites are still the main culprit.

3. Viral Hemorrhagic Fevers (Filoviruses)

Ebola outbreak

2015 Ebola outbreak | Photo: Carrie Nielsen / CDC

If any disease can cause fear in the 21st century it is the viral hemorrhagic fevers from the filovirus family. These include Ebola and Marburg viruses both of which have no effective treatment, no vaccine and a fatality rate of up to 90%. Combines with some very unpleasant symptoms these are potentially the deadliest viruses on earth.

In terms of diagnosis Marburg and Ebola are clinically indistinguishable. The name of this group of viruses is a big clue to some of the symptoms  – obviously a fever which is accompanied by pain throughout the body; joints, muscles, abdominal cramps, headaches. The hemorrhagic aspect comes from the way filoviruses interfere with the blood’s clotting mechanism resulting in the possibility of bleeding from each or every orifice. The more than likely death is generally due to multiple organ failure and internal tissue necrosis.

Ebola and Marburg had generally only occurred in isolated villages in Central Africa resulting in small outbreaks which rapidly burnt themselves out. However, in 2013 Ebola arrived in the West African country of Guinea where it was not recognised as such until it had time to spread rapidly. Over the next 2 years the Ebola epidemic raged across six countries infecting 25,000 people of which around a half died.

The largest outbreak of Marburg virus was in 2004 in Angola. Out of 252 infected 227 died, i.e. 90%. In an earlier epidemic in the Congo the fatality rate was 83%.

Both Marburg and Ebola are thought to have spread to humans from wild animals. Although the first reported cases of Marburg occurred in researchers working with African green monkeys it is believed that the natural host species are bats. This is also the case with Ebola which makes bats carriers of some of the most feared diseases on earth.


HIV / AIDS virus

HIV virions budding from an infected immune cell | Photo: CDC

Over the past three decades, since AIDS became headline news, people have become a little desensitized to what is a devastating disease. Huge progress in antiretroviral drugs have meant that given the right regimen of medicines infection with HIV is not the death sentence it once was.

The disease is yet another to have originated in Central Africa where it is believed to have been harboured in monkey populations for millions of years before crossing to humans in the mid 20th century. Exactly how this happened is not known but it is believed that the monkey version (SIV) was passed to humans via eating bushmeat – this later mutated into what we know as HIV.
It is suspected that HIV existed some time before it became headline news; the first recorded case was in the Congo in 1959 and there was what appeared to be a case in the U.S. in the mid 1960s.

HIV attacks the immune system which firstly stops it being destroyed and secondly makes the person susceptible to a wide range of very unpleasant infections. Once the immune system has been compromised beyond a certain level the disease is classified as AIDS. Without an effective immune system the body is at risk from a host of bacteria, viruses, fungi, and parasites. There are also a number of cancers that are prevalent in those with HIV. Among the most common causes of death for those infected with HIV are T.B. and pneumonia but there are any number of other complications.

The key reason it has been so difficult to find an outright cure for HIV is the fact that it is constantly and rapidly changing. It reproduces rapidly (around 10 billion new individual virions per day) and the mutation rate is high. Even in a single person the genetic variety of the virus can resemble an evolutionary tree with different organs infected with virtually different species.

Today, there are about 40 million people living with HIV, mostly in sub-Saharan Africa. Sadly, only around half of those infected have access to the required drugs so the global death rate from AIDS is still shockingly high. Estimates suggest there are nearly 2 million deaths every year and over the last 30 years the virus has claimed over 25 million lives.

1. Influenza

Spanish Flu

Influenza or flu as it is more commonly known is hardly the most exciting choice for a list of deadly viruses. Everybody gets flu and for the majority it is not particularly nice but no big deal. However, every year influenza racks up a massive number of deaths by picking off the vulnerable – the old, the very young and the sick. Despite there having been a safe and effective vaccine for over 60 years, influenza still causes up to half a million deaths annually.

But this is just the baseline figure, on top of this there are the occasional devastating epidemics where a particularly virulent strain of the virus develops. The 1918 “Spanish Flu” pandemic is a prime example of this. It is believed to have infected almost a third of the global population and caused up to 100 million deaths. During the epidemic the fatality rate was 20% of those infected compared to the usual 0.1% from seasonal flu. One reason the Spanish flu was so deadly was it killed healthy people; the particular strain caused the immune system to turn on itself creating an overreaction known as a cytokine storm. Therefore those with the strongest immune systems were at the greatest risk.

No other illness has even come close to figures like this and this is what makes influenza so dangerous. The flu virus has the ability to frequently combine and mutate to form new strains. At present the most deadly strains and the most contagious strains are thankfully distinct. One fear is that the a potentially lethal strain such as H5N1 bird flu, which is is not able to spread from person-to-person, would only require a small genetic event to create a possible epidemic. Although to date there have only been just over 600 cases of bird flu nearly 60% of these have been fatal making this one of the most virulent human diseases.