Here is a list of the ten viruses that cause the most human deaths globally per year. Details of the virus family and the diseases that the viruses provoke are also included.
All statistics on death rates are taken from World Heath Authority (WHO) virus data, unless otherwise stated.
# 10 Dengue
Global deaths per year: 25,000
BackgroundDengue fever, also known affectionately as ‘bonecrusher disease’, is an up-and-coming disease caused by one of four closely-related viruses.
The WHO estimates that a whopping 2.5 billion people (two fifths of the World’s population) are at risk from Dengue. It puts the total number of infections at around 50 million per year, and Dengue fever is now epidemic in more than 100 countries.
How it spreadsThe Dengue virus is transferred to humans through the bites of infective female Aedes mosquitoes. The virus circulates in the blood of an human for two to seven days, during the same time that they have the fever.
If an Aedes mosquito feeds on the infected human during this time, it may acquire the virus. Thus, infected humans are the main carriers and multipliers of the virus.
What it doesDengue fever is a severe flu-like illness characterised by severe headache, fever, muscle and joint pains, and a rash. The fever itself is rarely fatal.
However, Dengue hemorrhagic fever (DHF) can result as a complication of the illness, in which case the outlook is much grimmer.
This illness begins with a sudden rise in body temperature (up to 41 degrees), together with facial flush and other flu-like symptoms.
In severe cases, after a few days of fever, the temperature will plummet and signs of circulatory failure will arise. Finally, the patient may go into a state of shock and die within 12-24 hours.
TreatmentThere is no vaccine against Dengue, although several potential vaccines are in the early stages of development. Prevention of further epidemics is currently based on eradicating the spread of the mosquitoes, through vector controls and use of insecticides.
In terms of treating the symptoms of Dengue Fever, increased fluid intake is recommended in order to prevent dehydration, and intravenous fluids may be given to minimize this risk.
# 9 Yellow fever
Global deaths per year: 30,000
BackgroundThere have been several major yellow fever epidemics in recent history. Notable examples include Spain in the 19th Century, when 300,000 people died of the disease; and an outbreak during the Haitian revolution of 1802, when more than half the French army was wiped out by the virus.
The virus is still prevalent in many parts of the World, with West Africa seen as the home of yellow fever.
How it spreadsThe yellow fever virus is transmitted by mosquito bites. Once it has been transmitted, the virus replicates locally, before being transported to the rest of the body through the lymphatic system.
Once the virus has taken hold of the lymphatic system, it attacks organ systems, such as the heart, kidneys, adrenal glands, and the liver.
What it doesThe symptoms of yellow fever appear suddenly and intensely, after an incubation period of 3-5 days.
Within 24 hours of the onset of the toxic phase of the disease, the sufferer will develop some nasty symptoms, including fever, chills, bleeding into the skin, headache, back pains and slowed heartbeat.
Jaundice will kick in on the second or third day. Then, after another day or so, the symptoms will appear to recede.
However, just when you might think the patient was out of the woods, the symptoms will suddenly come back with a vengeance, increasing in severity and prompting internal hemorrhage. This is followed by delirium, coma, then finally, death (in about 50% of cases where the toxic phase presents itself). Not a pleasant experience at all.
TreatmentThere is no cure for yellow fever, though there is an effective vaccine that gives ten-year immunity from the virus.
# 8 Rabies
Global deaths per year: 55,000
BackgroundThe name for this virus is derived from the Latin word, rabies, meaning ‘madness’. It is famously transmitted by animals such as dogs, bats, foxes or monkeys.
Rabies is found throughout continental regions of Asia, Africa, and America and mainland Europe. Countries such as Great Britain, Japan, Australia, and most of Scandinavia are rabies-free, due to tight controls on importing animals.
How it spreadsRabies is a zoonotic virus transmitted through the bite of an animal. The virus worms its way into the brain along the peripheral nerves.
The incubation phase of the rabies disease can take up to several months, depending on how far it has to go to reach the central nervous system.
What it doesIf you contract rabies, be prepared for some strange experiences. The disease starts with relatively mild symptoms such as headache or fever. Once it really kicks in though, it provokes acute pain, violent movements, depression, uncontrollable excitement, and an inability to swallow water (rabies is often known as ‘hydrophobia’).
After these symptoms subside, the fun really starts, as the infected person experiences periods of mania, followed by coma, then death, usually caused by respiratory insufficiency.
TreatmentThere is a vaccine available to prevent infection with the rabies virus, and there are widespread immunization programs for both humans and animals.
Post-exposure prophylaxis is generally a successful way of treating rabies infections, if administered within 10 days of the infection appearing. Other treatments include induced coma and ketamine.
# 7 Hantavirus
Global deaths per year: 70,000 (NY Times Health Guides)
BackgroundNot all mice are as sweet and innocent as Mickey or Jerry. Meet one who is carrying a hantavirus, and you’ll soon realise that these cute little rodents can be as lethal as a tiger.
The virus is named after the Hantan River in Korea. Hemorrhagic fever with renal syndrome (HFRS) was first encountered by Western medicine during the Korean War in the early 1950s.
A new hantavirus-related disease called hantavirus pulmonary syndrome (HPS) was recognized in New Mexico in 1993, caused by the Sin Nombre virus.
How it spreadsHantaviruses are transmitted through aerosols of rodents' excreta, saliva and urine. Direct contact with rodents isn’t needed for transmission, and infection normally occurs through the inhalation of dust or dried particles that carry dried saliva or waste products from an infected rodent.
What it doesThe symptoms of HFRS can occur between one and six weeks after exposure. The disease starts with flu-like symptoms along with facial flushing. Around day four, a sudden and extreme albuminuria may occur, together with ecchymosis, scleral injection and bloodshot eyes.
Other symptoms include hypotension, shock, respiratory distress and/or failure, and renal impairment and/or failure.
HPS affects the lungs rather than the kidneys and is much more dangerous than HFRS. Most patients who develop HPS die within a few days of infection.
TreatmentHFRS can be treated with ribavirin, injected intravenously, to shorten the illness and reduce the risk of death. HPS, on the other hand, has no effective treatment as yet.
# 6 Measles
Global deaths per year: 197,000
BackgroundOne of the most prolific serial killers of all time, measles has done a pretty good job of exterminating populations throughout the ages. Its war on humanity dates back to 165-180AD, when it was believed to have wiped out a large part of the Roman army, and killed as much as one third of the population in some areas.
Other infamous outbreaks of the infection include one in Cuba in 1529, where two thirds of the natives were killed. Two years later, half the population of Honduras was destroyed by measles.
Over the last 150 years, the virus has been responsible for the deaths of around 200 million people.
How it spreadsMeasles is a highly-contagious virus that is spread by droplets, passed on through direct contact with an infected person.
The virus spreads through the respiratory route via aerosol droplets and respiratory secretions. These can remain infectious for a few hours. The upper respiratory tract, or conjunctiva, is the area where measles is acquired.
What it doesThe incubation period for measles is about ten to 14 days. After this time, cold-like symptoms such as a runny nose, cough, conjunctivitis, and fever start to appear.
A couple of days later, the inside of the cheeks start to break out in white spots. Soon after this, the body becomes a patchy mess of red blotches, and the victim may also suffer nausea, vomiting, abdominal pains and diarrhoea.
Measles is generally only fatal where other complications arise such as measles pneumonia, encephalitis, myocarditis, and secondary bacterial infections.
TreatmentChildren in developed countries such as the UK and USA are offered vaccination against measles at around the age of 12 months. When the infection does strike, a number of effective treatments are available, such as antiviral drugs, fever-reducing syrups and vitamin A.
# 5 Hepatitis C
Global deaths per year: 500,000 related (56,000 directly attributable)
BackgroundOne might think of Hepatitis C (HCV) as the “silent but violent” virus. An estimated 200-300 million people worldwide are infected with hepatitis C, a disease of the liver caused by the hepatitis C virus.
However, according to estimates, in some European countries more than 90% of people infected with HCV don’t even realise they have it. Most people who develop the disease only find out years after infection, when they start to develop signs of liver failure.
How it spreadsHepatitis C is spread through blood contact with another person. Infection can come through sharing drug-injecting equipment, blood transfusions or organ transplants, body piercings and tattoos, or dental exposure. The virus can also be transmitted sexually, though this is very rare.
The virus itself is a very small (50nm), single-stranded RNA virus.
What it doesMost people infected with hepatitis C don’t have any symptoms and feel fine for years. However, liver damage invariably rears its ugly head over time, often decades after first infection. In fact, 70% of those infected develop chronic liver disease, 15% are struck with cirrhosis and 5% can die from liver cancer or cirrhosis.
In the USA, hepatitis C is the primary reason for liver transplants.
TreatmentThe bad news is there is no vaccine against hepatitis C. Common treatment for the infection includes a combination of Pegasys and PEG-Intron, and the antiviral drug Ribavirin.
A number of immunoglobins have been developed to treat hepatitis C, and research is ongoing to improve their effectiveness.
# 4 Influenza
Global deaths per year: 500,000
BackgroundMuch has been made in the press of the recent ‘swine flu’ pandemic, caused by the spread of the H1N1 virus. But, the truth is, influenza has been a prolific killer for centuries. The symptoms of influenza were first described more than 2,400 years ago by Hippocrates.
Pandemics generally occur three times a century, and can cause millions of deaths. The most fatal pandemic on record was the Spanish flu outbreak in 1918, which caused between 20 million and 100 million deaths.
How it spreadsInfluenza viruses affect birds and mammals, and are transmitted through the air via coughs and sneezes. Infection can also be spread by bird droppings, blood, nasal secretions and saliva.
In order to invade a host, the virus shell includes proteins that bind themselves to receptors on the outside of cells in the lungs and air passages of the victim.
Once the virus has latched itself onto the cell, it takes over so much of its machinery that the cell dies.
Dead cells in the airways cause a runny nose and sore throat. Too many dead cells in the lungs cause death.
What it doesSymptoms of flu present themselves within a couple of days of infection. Fever is a common symptom at first, with body temperature rising as high as 39 degrees. Other symptoms include head and body aches, irritated eyes, fatigue and extreme coldness.
Most people will completely recover from flu within one or two weeks. However, for the elderly and those with certain chronic health problems, the disease can be deadly.
People with existing respiratory conditions, or with a weak immune system, are especially at risk of death.
TreatmentVaccinations against flu are common in developed countries. However, a vaccination that is effective one year may not necessarily work the next year, due to the rate at which a flu virus evolves, and to the fact that new strains will soon replace older ones.
Antiviral medication can be used to treat influenza once it has presented itself.
# 3 Hepatitis B
Global deaths per year: 521,000
BackgroundIf there’s one thing the hepatitis B (HBV) virus loves, it’s human contact. Incredibly, around a third of the World’s population (over 2 billion people) has had the pleasure of the company of this nasty virus, including 350 million chronic carriers.
In China and other parts of Asia, up to 10% of the adult population is chronically infected.
How it spreadsHepatitis B is spread from exposure to infected blood. Transmission can occur through unprotected sexual intercourse, blood transfusions, re-used needles and syringes, and mother-to-baby vertical infection.
The virus enters the body by binding to an unknown receptor on the surface of a cell, then entering it via endocytosis.
What it doesThe symptoms of acute hepatitis B are not pretty – yellowing of the skin of eyes, dark urine, vomiting, nausea, extreme fatigue, and abdominal pain.
Luckily, more than 95% of people who contract the virus as adults or older children will make a full recovery and develop immunity to the disease. In other people, however, hepatitis B can bring on chronic liver failure, due to cirrhosis or cancer.
TreatmentAcute hepatitis B has no specific treatment, other than making sure the patient is comfortable, and adequately replacing fluids.
Chronic hepatitis B can be treated with drugs such as interferon and anti-virals.
Liver cancer is almost always fatal, but can be treated with chemotherapy to prolong life. Cirrhosis of the liver can often be cured via a liver transplant.
# 2 Rotavirus
Global deaths per year: 611,000 (NCBI)
BackgroundDiarrhoea is no joke, especially when the rotavirus is involved. According to the WHO, this merciless virus causes the deaths of more than half a million children every year. In fact, by the age of five, virtually every child on the planet has been infected with the virus at least once.
Immunity builds up with each infection, so subsequent infections are milder. However, in areas where adequate healthcare is limited, the disease is often fatal.
How it spreadsAs gross as it sounds, rotavirus infection usually occurs through ingestion of contaminated stool. Because the virus is able to live a long time outside the host, transmission can occur through ingestion of contaminated food or water, or by coming into direct contact with contaminated surfaces, and then putting hands in the mouth.
Once it’s made its way in, the rotavirus infects the cells that line the small intestine and multiplies. It emits an enterotoxin, which gives rise to gastroenteritis.
It’s worth pointing out that diseases caused by rotavirus generally only occur in children.
What it doesNot everyone who gets infected with rotavirus exhibits symptoms, but if they do occur, they normally start suddenly and messily.
Common symptoms include vomiting, upset stomach, watery diarrhoea, severe dehydration, high fever, loss of appetite and mucus in stools.
TreatmentTreatment of rotavirus infection varies depending on the symptoms. The most important part of the treatment is the maintenance of hydration, usually through plain water, a solution of water with salts and sugar, or an intravenous drip.
If untreated, children can die from dehydration resulting from gastroenteritis.
# 1 Human immunodeficiency virus (HIV)
Global deaths per year: 3.1 million
BackgroundThe mother of all viruses, HIV has claimed the lives of more than 25 million people since 1981. The virus is thought to have originated in primates in sub-Saharan Africa and moved on to humans in the early Twentieth Century.
A third of all AIDs deaths occur in sub-Saharan Africa and, according to recent estimates, HIV will affect 90 million people in Africa, leaving 18 million orphans.
How it spreadsHIV is transmitted through contact with infected bodily fluids. The virus can be found in varying concentrations in blood, semen, vaginal fluid, breast milk, saliva, and tears.
The main ways infection occurs is through unprotected sexual intercourse, exchange of needles, breast milk, and vertical transmission from mother to baby at birth.
HIV gets to the immune system by infecting important cells, including helper cells called CD4+ T cells, plus macrophages and dendritic cells.
Once the virus has taken hold, it systematically kills these cells, damaging the infected person’s immunity and leaving them more at risk from infections.
What it doesThe majority of people infected with HIV go on to develop AIDS, and this generally occurs between 10 and 15 years after, although it can be much sooner.
Early signs of AIDS include sudden weight loss, recurring respiratory infections, skin rashes and mouth ulcers.
Once a patient has AIDS, common infections and tumours that are normally controlled by the CD4+ T cells, start to affect the person.
In the latter stages of the disease, pneumonia and various types of herpes can infect the patient and cause death.
TreatmentThere is currently neither a vaccine nor a cure for HIV or AIDS. The only effective way of eradicating the disease is through prevention.
HIV can be treated using anti-retroviral drugs, in order to keep the amount of the virus in the body at a low level. This prevents weakening of the immune system and can help stop people from becoming ill for many years.
* Images courtesy of Public Health Image Library