1796 – Edward Jenner’s Smallpox Vaccine:

In 1796, Edward Jenner conducted a groundbreaking experiment that laid the foundation for modern vaccination. Jenner, a British physician, observed that milkmaids who had been infected with cowpox, a disease similar to smallpox, seemed immune to smallpox. Building on this observation, he developed a vaccine by inoculating a young boy with material from a cowpox sore and later exposing him to smallpox. The successful outcome of this experiment demonstrated the principle of vaccination—using a milder form of a pathogen to confer immunity to a more severe form—and became the basis for the eventual eradication of smallpox.