> The smallpox vaccine is the first vaccine to have been developed against a contagious disease. In 1796, British physician Edward Jenner demonstrated that an infection with the relatively mild cowpox virus conferred immunity against the deadly smallpox virus. Cowpox served as a natural vaccine until the modern smallpox vaccine emerged in the 20th century. ...

> The term vaccine derives from the Latin word for cow, reflecting the origins of smallpox vaccination.

https://en.wikipedia.org/wiki/Smallpox_vaccine