What is the most deadly sickness?

There are many deadly sicknesses, but the one that has caused the most deaths in human history is smallpox. Smallpox is a highly contagious infectious disease caused by the variola virus. It is characterized by a rash of small red bumps that can spread over the entire body. Smallpox was responsible for an estimated 300-500 million deaths in the 20th century alone. The disease was eradicated in 1980 thanks to a global vaccination campaign.