Influenza
From the gruesome flu pandemic that killed an estimated 50 million to 100 million people in 1918, to the 2009 swine flu pandemic that took thousands of lives, different strains of influenza viruses have caused some of the deadliest outbreaks of the past century. This group of pathogens is one of the most closely watched due to its enormous pandemic threat.
In its seasonal epidemics, the virus infects up to 15 percent of the population it hits. Annual epidemics are thought to result in between 3 million and 5 million cases of severe illness and between 250,000 and 500,000 deaths every year around the world, according to the World Health Organization (WHO).
The ever-changing virus has gone through several major mutations in the past century and has been able to repeatedly spread to humans via domestic animals. Significant outbreaks caused by the virus include the Asian flu in 1957 and the Hong Kong flu in 1968, which each caused several million deaths worldwide, and the swine flu in 2009.