Smallpox Vaccine — Edward Jenner (1796)

The smallpox vaccine is the most consequential breakthrough in medical history, as it enabled the complete eradication of a human disease—a feat never achieved before or since. Edward Jenner's 1796 experiment using cowpox matter to inoculate an eight-year-old boy established the principle of vaccination and sparked a global campaign that, by 1980, eliminated smallpox after it had killed an estimated 300 million people in the 20th century alone.