In 1796, English physician Edward Jenner performed a daring experiment that led to the world's first vaccine. Noticing that milkmaids seemed immune to smallpox after catching the milder cowpox, Jenner inoculated an eight-year-old boy, James Phipps, with pus from a cowpox lesion. When later exposed to smallpox, the boy did not fall ill. This proved that a controlled infection with a related, weaker pathogen could provide immunity against a deadly disease. The word 'vaccine' itself comes from the Latin 'vacca,' meaning cow. Jenner's discovery eventually led to the total global eradication of smallpox in 1980 and laid the groundwork for modern immunology. His work is estimated to have saved more lives than that of any other human in history.