The world’s deadliest virus
Which one poses the greatest threat?
In recent years, viruses have been emerging as if from nowhere. At least, that’s how it seems. Media reports on outbreaks of killer viruses like SARS, swine flu, and West Nile virus have raised international fear. Currently, the virus dominating news headlines is a new strain of bird flu (H7N9) in China. Detected last March, the virus transmits easily from birds to humans, killing about one in five people it infects. “This is an unusually dangerous virus for humans,” a World Health Organization official stated during a press conference held at the end of April in Beijing. “Definitely one of the most lethal influenza viruses that we’ve seen so far.”
Other current fears centre around a novel coronavirus (NCoV), a SARS-like virus that has killed about 20 people since 2012. News organisations have been closely watching the creeping count of cases and deaths. Our fear of viruses is clear: which one has the capacity to become a pandemic we can’t control? In a world of global air travel, a deadly virus could sweep round the earth with frightening speed.
The thought of protein-coated fragments of genetic material entering the body silently and invisibly hijacking our own cells is unnerving. Viruses outnumber all other life forms on earth. Their ubiquity becomes apparent when we look at the numbers: scientists have estimated that there are about 10[31 ]viruses on our planet—10 million times the number of stars in our universe. In fact, there are estimated to be 100 times as many virus particles in the human body as there are human cells.
The public and public health officials alike fear the emergence of a deadly new virus, but how do we define deadly? Important criteria could include the case fatality rate, the basic reproductive rate (a way to measure the transmission potential), and the incubation time (boxes 1-3). The most feared viruses are the ones that kill most of those infected, like rabies, which has a case fatality rate of 100% when untreated, or the Ebola virus, which has a case fatality rate of up to 90%. Yet a virus that kills its host more quickly than it can spread will die out before infecting a significant proportion of the population. Ebola virus has caused just 1300 deaths to date, while the 1918 Spanish flu (with a case fatality rate of around 2.5%) led to an estimated 50 million deaths worldwide.
Box 1: Deadliest viruses by death rate
Over the past century, urbanisation, changing sexual behaviour, a growing world population, and increased mobility have helped viruses to spread. Here are seven deadly viruses that claim thousands—sometimes even millions—of lives each year.
HIV/AIDS causes a staggering 1.6-1.9 million deaths every year. Over the past three decades, the virus has claimed more than 25 million lives. HIV-like viruses (lentiviruses) arose in African primates 5-12 million years ago but appeared in the human population only in the 20th century. Today, there are about 34 million people living with HIV, most of them in sub-Saharan Africa (69%). Only around half of eligible patients have access to antiretroviral therapy.
It has been estimated that hepatitis (all types) causes around a million deaths annually. Hepatitis B accounts for 600 000 deaths and is 50-100 times more infectious than HIV. Additionally, every year there are 350 000 deaths related to hepatitis C and 70 000 deaths related to hepatitis E.
Despite the fact that effective and safe vaccines have been available and used for more than 60 years, influenza causes 250 000-500 000 deaths every year. Elderly people and those with chronic medical conditions are at particular risk of developing serious illness.
Although most infections with human papillomavirus (HPV) are not symptomatic, persistent infection can cause cervical cancer. In fact, HPV is the cause of virtually all cervical cancers (99%), which is the second most common cancer in women worldwide. HPV causes 275 000 deaths every year.
More than 55 000 people die of rabies every year. Without post-exposure prophylaxis (PEP), this vicious zoonotic disease—which causes acute encephalitis—is fatal within days of the onset of symptoms. In 2005, doctors reported the case of a 15 year old girl who became the first person to survive rabies without previous vaccination after being treated with the “Milwaukee protocol.”
In recent years, the incidence of this mosquito-borne viral infection has increased dramatically. Around 50-100 million people are infected every year, resulting in 12 500-25 000 deaths. The rapid spread of its mosquito carrier has been partly attributed to increased globalisation, although the African slave trade gave it an early push. Epidemics became common during and after the second world war.
Rotavirus is the most common cause of severe diarrhoea in infants and children globally. Each year there are around 500 000 deaths related to rotavirus. Vaccines against the virus are available and constitute an important measure in reducing mortality in vulnerable groups, such as children aged under 5 in low income countries.
Another important determinant of deadliness is the mode of transmission. A perpetual fear is that deadly viruses will mutate to develop person to person transmission, allowing rapid spread (rather than, for example, spreading to humans from an infected animal source). The WHO’s recent admission that the novel coronavirus is probably spreading from person to person made the news for this reason.
Box 2: Deadliest viruses by fatality rate
Ebola virus, named after the Ebola river in the Congo where the virus was first found, causes highly lethal haemorrhagic fever. The lack of a cure drives fatality rates up to 90%. Outbreaks often occur in remote villages near rainforests, so the disease typically does not spread far. The virus has been mainly restricted to Africa, although a few laboratory incidents (the latest in 2004 in Russia) have led to deaths outside the continent.
Clinically indistinguishable from Ebola (both are filoviruses), the Marburg virus causes abrupt illness with severe haemorrhagic manifestations within a week of the onset of symptoms. Fatality rates have ranged from 24% to 88%. The disease was first described in 1967 in the German city Marburg, when several laboratory workers were admitted to hospital with a severe and mysterious disease after working with a species of African green monkey for polio vaccine research.
Lassa fever is responsible for around 5000 deaths annually. The extremely virulent illness is endemic in West Africa—it is one of the most prevalent haemorrhagic fevers in that region. The fatality rate is typically estimated at 15-20%, but some studies suggest a mortality of up to 50%. The virus infects almost every tissue in the human body and can be contracted through exposure to rodents.
The genus Henipavirus includes three members: Hendra virus (HeV), Nipah virus (NiV), and Cedar virus (CedPV). The latter was identified in mid-2012, and, with no recorded human cases, is thought to be harmless. Hendra virus and Nipah virus, however, are highly virulent zoonotic pathogens with a mortality rate between 50-100%. Harboured by fruit bats, the viruses are not considered highly contagious. They were first discovered in the 1990s when several deadly outbreaks occurred in Australia and Malaysia.
This genus contains the rabies virus (nearly 100% fatality rate) and closely related viruses including Duvenhage virus, Mokola virus, and Australian bat lyssavirus. Although rare—only a small number of cases have been confirmed and documented—these infections are nearly always fatal. Bats are known to be animal vectors for all types except Mokola virus (which has been isolated mainly in small mammals).
In late 2011, two research teams caused international controversy when they genetically altered the H5N1 avian flu virus in their laboratory, making it capable of airborne transmission between mammals (and thus potentially allow for human to human transmission). The lead author of one of the studies, virologist Ron Fouchier, described the altered virus as “probably one of the most dangerous viruses you can make.” The development of the highly contagious virus caused an uproar in the scientific community, leading to an intense debate about whether such results should ever be released. Some researchers supported the work, arguing that these results would help develop countermeasures against a future flu pandemic; others were concerned about biosecurity, warning that this information could be turned into a weapon by a rogue scientist.
Box 3: Deadliest viruses by total deaths
The history of humans can be seen as a continuous battle for survival against microbes that have the ability to evolve much more quickly than us. In ancient times, the human population was not large enough for pandemics to occur. Changes in farming methods and agriculture, as well as increasing population densities and the formation of cities, contributed to the spread of viruses. Since then, infectious attacks have regularly changed the course of history. Below are four deadly viruses that have had a large impact on history.
Smallpox emerged around 10 000 BC and has been one of the most devastating human diseases throughout history. The highly contagious disease, caused by infection with variola virus, has led to various disastrous epidemics, starting in ancient Egypt and moving on to India, China, and Europe. Voyages from Europe to the New World introduced the virus there too, devastating native American populations. Estimates indicate fatality rates of 80-90% during epidemics. The disease also killed around 50% of the native population of Australia during the early years of British colonisation, making it the principal cause of death in Aboriginal populations between 1780 and 1870.
It is estimated that smallpox was responsible for 300-500 million deaths during the 20th century. In 1796 Edward Jenner developed the first vaccine for smallpox and after successful vaccination campaigns the WHO certified the eradication of smallpox in 1979.
The Persian physician Rhazes (860-932 AD) was the first to give a scientific description of measles. Historically, this highly contagious disease was prevalent throughout the world, causing many millions of deaths. The Antonine plague from 165-180 AD was probably caused by measles (or smallpox) brought back to the Roman Empire by troops returning from the Near East. The total death rate has been estimated at five million. Even today, with safe and cost effective vaccines available, measles is one of the leading causes of death among young children. The WHO reports that there were 158 000 deaths from measles in 2011.
The 1918 pandemic infected a third of the world’s population and caused 100 million deaths worldwide, many more than the 20 million who died in the first world war. Unusually, this flu mostly killed young adults. This is thought to be because the virus killed by a “cytokine storm”—an immune overreaction—meaning that it was deadliest in those with the strongest immune systems. The virus crossed the globe before vanishing completely within 18 months. Around the time of the flu pandemic children would skip rope to the rhyme:
I had a little bird,
And its name was Enza.
I opened the window,
This acute haemorrhagic disease has caused several devastating epidemics in the past, including those in the United States and Spain during the 18th and 19th century (president George Washington fled from Philadelphia, together with about half of the residents, during one of the largest epidemics of yellow fever, which killed around 10% of the population). The virus is not eradicated yet; there are an estimated 200 000 cases of yellow fever worldwide, killing 30 000 people each year.
What does the future hold for our battle against viruses? Their resilient nature makes eradication seem unlikely, so we will need to keep innovating. To conclude with the ominous words of Nobel prize-winning molecular biologist Joshua Lederberg: “The single biggest threat to man’s continued dominance on the planet is the virus.”Shannon Plaxton, Clegg scholar, BMJ
1BMJ, London UK
Correspondence to: firstname.lastname@example.org
Competing interests: None declared.
Provenance and peer review: Commissioned; not externally peer reviewed.
Cite this as: Student BMJ 2013;21:f3393