The first vaccine used cowpox to beat smallpox

English physician Edward Jenner noticed that dairymaids who caught cowpox, a mild disease from cattle, seemed protected against the far deadlier smallpox. To test the idea, on 14 May 1796 he took matter from a cowpox sore on the hand of milkmaid Sarah Nelmes and inoculated an eight-year-old boy, James Phipps.

Phipps developed only a mild reaction. When Jenner later exposed him to smallpox material, no disease followed — the protection held. It was the first deliberate vaccination of its kind.

Jenner named the procedure after the Latin word for cow, vacca — the root of the word “vaccine” we still use today. His method spread worldwide and, nearly two centuries later, underpinned the campaign that eradicated smallpox entirely.