Against which virus did Edward Jenner develop a vaccine?

Edward Jenner developed the smallpox vaccine in 1798, making him known as the "Father of Immunology". Jenner observed that milkmaids who had contracted cowpox, a similar but much milder disease than smallpox, did not seem to be susceptible to smallpox. Based on this observation, Jenner hypothesized that cowpox could be used to protect against smallpox. He tested this hypothesis by inoculating a young boy with cowpox and later exposing him to smallpox, proving that the boy was immune to smallpox.

The smallpox vaccine was a significant breakthrough in medical history, as smallpox had been a highly contagious and deadly disease for centuries, causing widespread epidemics and claiming millions of lives. The vaccine was eventually adopted by governments worldwide, and through mass vaccination campaigns, smallpox was eradicated from the world in 1980.