Twenty years later, the country doctor Edward Jenner extracted pus from a blister on the hand of a milkmaid and scraped it into the arm of an eight-year-old boy. The boy got a fever but did not become ill. Jenner then exposed the boy to smallpox, which did not infect him. Emboldened, Jenner continued his experiment on dozens of other people, including his own infant son. Before long, the procedure would be known by Jenner’s term for cowpox, variolae vaccinae, from the Latin vacca for cow, the beast that would forever leave its mark on vaccination.
