World's deadliest viruses and where COVID-19 fits in this scheme

The ebola outbreak of 2014 in West Africa lasted two years and claimed as many as 11,325 lives in Liberia, Guinea, Sierra Leone, Nigeria, Mali and the USA, infecting nearly 30,000 people.

Although not as common as other viruses, ebola has a high mortality rate, killing nearly 50% of those infected, according to the World Health Organization (WHO). Since its discovery in the Congo (then Zaire) in 1976, this was the worst ever outbreak of the disease which was believed to have been transmitted by bats. Known to be present through five different strains of the virus, the Zaire ebola virus is the deadliest with a 90% mortality rate. The ebola virus disease spread through touch and through sexual intercourse, and was also transmitted from the mother to a newborn child (also known as vertical transmission).

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While swine flu is believed to have originated in 1918 when there was a connection established with the disease appearing in pigs, the 2009 pandemic was its deadliest occurrence in recent memory.

Caused by a strain of the H1N1 virus that is also the carrier for the common flu and connected with the Spanish flu outbreak of 1918, this one appeared to be a new strain of the same virus. The 2009 outbreak originated in Mexico, and the WHO claims the outbreak led to more than 18,000 deaths, and estimated that between 700 million and 1.4 billion people contracted the infection during that time.

The HIV/AIDS global pandemic, first cases of which were brought to light in the early 1980s, originated in Africa. This viral disease is caused by the human immunodeficiency virus (HIV), which is believed to have come from a species of monkeys and spread through unprotected sex, blood transfusions, using contaminated needles, as well as from an infected mother to a child via vertical transmission.

Until 2018, the disease is believed to have claimed over 32 million lives worldwide (according to the WHO) with an estimated 38 million living with HIV, a majority of them in Africa.

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Multiple strains of the virus causing bird flu have frequently caused panic around the world, the latest of which occurred in China in 2013 through the H7N9 strain of the virus. More commonly referred to as bird flu, the first avian influenza outbreak among humans occurred in 1997. It was caused by the H5N1 strain of the highly pathogenic influenza virus, and has a high mortality rate of over 60% among humans.

Unlike other forms of flu, however, avian flu didn't have high cases of human transmission, and was instead carried from birds, particularly livestock where humans come in close contact while dealing with poultry and meat.

In late 2002, another animal virus, purported to have come from bats, infected humans in the Guangdong province of China, which was later identified to be another strain of the coronavirus, codenamed SARS-CoV, and later called severe acute respiratory syndrome, or SARS.

The pandemic spread to 26 countries in no time, infecting over 8,000 people and killing another 774. It was the first strain of coronavirus identified by scientists. Although the outbreak was limited to a few months, the fatality rate was a high 11%.

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A tropical disease caused by the bite of mosquitoes carrying the dengue virus, dengue fever is common across 120 countries, a majority of which are situated in Asia and South America. It infects nearly 400,000 people every year seasonally and as many as 40,000 die annually because of it. Although treatable, the mortality rate can shoot up to almost 20% if a patient goes untreated.

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Classified as one of the deadliest viral diseases known, the Marburg virus (MARV) causes haemorrhagic fever and is also known to have been transferred to humans by fruit bats, showing similar symptoms to that of ebola. The first known case was discovered in the 1960s in the German town of Marburg, which it is named after. Although a sizable outbreak only took place once between 1998 and 2000 in the Congo, killing 128 out of the 154 infected, the fatality rate is extremely high.

Read more: Crimean-Congo Haemorrhagic Fever

A recent case of hantavirus in China sent panic waves in the global community already jostling with the COVID-19 pandemic. The orthohantavirus is also present through various strains that are airborne and carried by different species of rats, and hasn't had any sign of being transmitted from one human to another.

The first case of hantavirus emerged among soldiers during the Korean War in the 1950s. Haemorrhagic fever and hantavirus pulmonary syndrome have been the two main diseases caused by this type of virus.

The only disease in the list to have been successfully eradicated, smallpox was caused by the variola virus that led to the formation of sores in the mouth, followed by bumps filled with fluid, and spread through human contact or contaminated surfaces and objects.

At least 300 million people are believed to have died from smallpox in the last century alone, and the disease clearly had a high mortality rate, killing a third of those infected.

Poliomyelitis, commonly known as polio, is caused by the poliovirus which caused large outbreaks around the world throughout the 20th century. As many as 350,000 people were affected by the disease every year in the 1980s, but concerted efforts into developing a vaccine has led to its eradication from most countries in the world. The polio vaccine is now administered to newborns, infants and children to prevent the disease from ever plaguing them.

Read more: Vaccines for newborns, infants and children

The influenza virus may well be the most cunning of them all, as it continues to mutate into various strains and versions even today, the latest of which has led to the global pandemic through its new strain of coronavirus. COVID-19 is the latest outbreak of an influenza type pandemic. The Spanish flu of 1918 wiped out a third of the global population in just two years, while recent outbreaks of swine flu and Asian flu have also kept the global health community on its toes.

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