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Coronavirus or COVID-19 has caused thousands of deaths worldwide and infected millions. It's not the first-time humans have been battling with viruses as many deadly viruses in history have created distress across the globe and killed millions.
For centuries, we fought viruses like Ebola and influenza and won the battles. But sometimes, their new strains which have evolved over time and became even more dangerous to humans come back to threaten the world. The outbreak of COVID-19 is a reminder to mankind that we have fought before and will have to fight again. Here is the list of deadly viruses in human history.
Ebola virus disease is regarded as a deadly disease and it has infected people from time to time. It was first discovered in the year 1976 in Central Africa, and since then it has affected several African countries. Studies suggest that Ebola virus is animal-borne and has come from bats and nonhuman primates such as monkeys and chimpanzees. Ebola has been infecting humans for 40 years to date. Symptoms of Ebola virus include fever, diarrhoea, abdominal pain, unexplained haemorrhaging, weakness and bleeding.
Outbreak year: 1976, 1977, 1979, 1989, 1990, 1992, 1994, 1995, 1996, 2000, 2001, 2003, 2004, 2005, 2007, 2008, 2011, 2012, 2014, 2017, 2018
Average mortality rate: 50 per cent
It is a family of viruses that spread mainly by rodents such as cotton rats and deer mouse. The infection is spread through an aerosolised virus that shed mainly in faeces, urine or saliva of the infected host. Hantavirus was first recognised in the year 1993 in the southwestern United States. Symptoms of hantavirus include high fever, body ache, chills, breath shortness, fluid in the lungs and haemorrhagic fever with renal syndrome.
Outbreak year: 1993, 2012, 2017, 2020
Average mortality rate: 1-15 per cent
The outbreak of Severe Acute Respiratory Syndrome or SARS occurred in November 2002 in Southern China but was identified first in the year 2003. It is a zoonotic disease that has spread through bats and civet cats. The latest COVID-19 is believed to be similar to SARS-CoV. It spread mainly through human-to-human transmission via respiratory secretions and faeces. Symptoms of SARS-CoV include shortness of breath, diarrhoea and severe respiratory diseases.
Outbreak year: 2002, 2003, 2004
Average mortality rate: 0- 50 per cent depending on the age group 
It is a fatal and neglected disease that spreads mainly through the bite or scratch of wild animals such as foxes, raccoons, bats, dogs and skunks. When these wild animals bite, the rabies virus infects the central nervous system and without proper treatment, the virus spreads to the brain and cause death. The first record of rabies was found in 2300 BC in the Mosaic Esmuna Code of Babylon. For the last 100 years rabies is aroundhumans.
Outbreak year: 1273-1562, 1429-1655, 1491-1689, 1531-1718, 1642-1782 
Average mortality rate: Approx. 99 per cent but preventable by vaccine
It is regarded as the worst viral disease in the history of Mexico. According to a journal Emerging Infectious Diseases, Cocoliztli first occurred in the mid- 16th century and was transmitted by rodents. The disease became aggravated due to extreme drought in Mexico and killed around 17 million people. Symptoms of Cocoliztli include haemorrhagic fever, black tongue, dark urine, neurologic disorders, bleeding from eyes and nose and ultimately death. 
Outbreak year: 1520, 1545, 1576, 1736 and 1813
Average mortality rate: 15 per cent
It is an infectious and highly contagious disease that mainly causes infection of the skin, respiratory system and immune system. Measles is caused by the measles virus which is closely related to the pathogen of cattle. The virus usually infects children and spreads by respiration or aerosol. It was first reported in the year 1765 in the US. According to the WHO, the vaccine of measles was first introduced in 1963 and until then, the death rate was 2.6 million per year. 
Outbreak year: Between the 11th-12th century
Average mortality rate: 10-30 per cent 
Other Deadly Viruses
7. Bird Flu
It is caused by the influenza virus and transmitted to humans by sick birds like wild duck or chicken.
Outbreak year: 1997
Average mortality rate: 60 per cent
It is a contagious disease caused by the variola virus. Smallpox is believed to have come from the African rodent poxvirus.
Outbreak year: 10000 BC
Average mortality rate: 30 per cent
It is a mosquito-borne disease caused by dengue virus. Dengue is transmitted by the bite of female Aedes mosquito.
Outbreak year: 1635
Average mortality rate: 20 per cent when left untreated
10. Spanish Flu
It is regarded as one of the deadly influenza pandemics. Spanish flu has originated from the gene of the avian origin or through birds.
Outbreak year: 1918
Average mortality rate: 10-20 per cent
11. Machupo Virus
It is an infectious disease caused by rodents. Machupo disease has caused epidemic of haemorrhagic fever in South and Central America.
Outbreak year: 1952
Average mortality rate: 5-30 per cent
Human Immunodeficiency virus is widely believed to have crossed species from chimpanzees to humans. Still, there's no cure for HIV.
Outbreak year: 1920
Average mortality rate: 10-11 per cent
It usually infects infants and young children. Rotavirus is caused by domestic and wild animals like bats, rodents and birds.
Outbreak year: 1973
Average mortality rate: 3.4 per cent
It is an infectious disease that causes paralysis to humans. Researchers believe that polio had come from monkeys.
Outbreak year: 1894
Average mortality rate: 5-10 per cent
15. Swine Flu
It is a disease caused by swine influenza viruses. Swine is caused by H1N1 strain which is found in pigs. It has caused a pandemic in the year 2009.
Outbreak year: 2009
Average mortality rate: 1-7 per cent