Since many years, even before our species evolved into its present form, there has been a battle between humans and viruses. Though vaccines and antiviral drugs have been developed for some of these to prevent infections from spreading widely and help sick people recover, there are still others for which we are a long way from winning the fight. Let us have a look at some of these killers, based on the likelihood that a person will die if they are infected with one of them, the sheer numbers of people they have killed, and whether they represent a growing threat.
Ebola spreads through contact with blood or other body fluids, or tissue from infected people or animals. The known strains vary dramatically in their deadliness. One strain, Ebola Reston, does not even make people sick, but the fatality rate of Bundibugyo strain is up to 50%, and for the Sudan strain it is up to 71%, according to World Health Organization (WHO).
The first known Ebola outbreaks in humans struck simultaneously in the Sudan and the Democratic Republic of Congo in 1976. There was another outbreak in West Africa began in early 2014, and is the largest and most complex outbreak of the disease to date, according to WHO.
Marburg virus is similar to Ebola in that both can cause hemorrhagic fever, meaning that infected people develop high fevers and bleeding throughout the body that can lead to shock, organ failure, and death.
Scientists identified Marburg virus in 1967, when small outbreaks occurred among lab workers in Germany who were exposed to infected monkeys imported from Uganda. The mortality rate in the first outbreak was 25%, but it was more than 80% in the 1998-2000 outbreaks in the Democratic Republic of Congo, as well as in the 2005 outbreak in Angola, according to the WHO.
The rabies virus destroys the brain, making it a really bad disease. The vaccine and antibodies that work against rabies are available. So, if you get bitten by a rabid animal, you can be treated. But if you donot get the treatment, there is a 100% possibility that you will die.
Although rabies vaccines have been introduced for pets since the 1920s, they have helped make the disease exceedingly rare in the developed world, but this condition remains a serious problem in India and parts of Africa.
This may be the deadliest virus of all in the modern world. An estimated 36 million people have died from HIV since the disease was first recognized in the early 1980s. It is still the one that is the biggest killer.
Powerful antiviral drugs have made it possible for people to live for years with HIV. But the disease continues to devastate many low- and middle-income countries, where 95% of the new HIV infections occur. According to WHO, nearly 1 in every 20 adults in Sub-Saharan Africa is HIV-positive.
Humans battled smallpox for thousands of years, and the disease killed about 1 in 3 of those it infected. It left survivors with deep, permanent scars and, often, blindness. In the 20th century alone, smallpox killed 300 million people. It was a huge burden on the planet and this spurred the campaign to eradicate from the Earth.
In 1980, the World Health Assembly declared the world free of smallpox.
Hantavirus pulmonary syndrome (HPS) first gained wide attention in the U.S. in 1993. More than 600 people in the U.S. have now contracted HPS, and 36% have died from the disease, according to the Centers for Disease Control and Prevention. The virus is not transmitted from one person to another, rather, people contract the disease from exposure to the droppings of infected mice.
According to a 2010 paper in the journal Clinical Microbiology Reviews, a different hantavirus caused an outbreak in the early 1950s, during the Korean War. More than 3,000 troops became infected, and about 12% of them died.
The most deadly flu pandemic, sometimes called the Spanish flu, began in 1918 and sickened up to 40% of the world's population, killing an estimated 50 million people.
During a typical flu season, up to 500,000 people worldwide die from the illness, according to WHO. But occasionally, when a new flu strain emerges, a pandemic results with a faster spread of disease and, often, higher mortality rates.
Scientists predict that if a new influenza strain finds its way in the human population,and can be transmitted easily between humans,it will cause severe illness and be a big problem.
According to WHO, dengue sickens 50 to 100 million people a year. Although the mortality rate for dengue fever is lower than some other viruses, at 2.5%, the virus can cause an Ebola-like disease called dengue hemorrhagic fever, and that condition has a mortality rate of 20% if left untreated.
Dengue virus first appeared in the 1950s in the Philippines and Thailand, and has since spread throughout the tropical and subtropical regions of the globe. Up to 40% of the world's population now lives in areas where dengue is endemic, and the disease, with the mosquitoes that carry it, is likely to spread farther as the world warms.
Dengue virus is a real threat and needs to be worked upon as there is no current vaccine against dengue. However, large clinical trials of an experimental vaccine developed by French drug maker Sanofi have shown promising results.
Rotavirus is the leading cause of severe diarrheal illness among babies and young children. The virus can spread rapidly through the fecal-oral route.
Although children in the developed world rarely die from rotavirus infection, the disease is a killer in the developing world, where rehydration treatments are not widely available. The WHO estimates that worldwide, 453,000 children younger than age 5 died from rotavirus infection in 2008.
Two vaccines are now available to protect children from rotavirus. The countries that have introduced the vaccine have reported sharp declines in rotavirus hospitalizations and deaths.