The world's first successful vaccine is the smallpox vaccine, developed by Edward Jenner in 1796. Jenner discovered that milkmaids who had contracted cowpox, a disease similar to smallpox but much milder, seemed to be immune to smallpox. He conducted an experiment where he inoculated a young boy with cowpox, later exposing him to smallpox, and found that he did not become ill. This groundbreaking work laid the foundation for immunology and vaccination, eventually leading to the worldwide eradication of smallpox by 1980, demonstrating the power of vaccination in public health.