The first vaccine was derived from a mild virus found on the hands of milkmaids

In 1796, physician Edward Jenner observed that milkmaids who contracted cowpox—a mild disease caught from cattle—never succumbed to the much more lethal smallpox. To test this theory, Jenner took pus from a cowpox lesion on a milkmaid named Sarah Nelmes and inoculated the arm of an eight-year-old boy. When the boy was later exposed to smallpox, he showed no symptoms, proving that the milder virus had 'primed' his immune system.