Vaccinations were pioneered in the 1790s by an English doctor named Edward Jenner who noticed that many milkmaids were immune to smallpox. They had all been infected by a form of pox that afflicts cows but is harmless to humans, and Jenner surmised that the cowpox had given the milkmaids immunity to smallpox. So he took some pus from a cowpox blister, rubbed it into scratches he made in the arm of his gardener’s eight-year-old son, and then (this was in the days before bioethics panels) exposed the kid to smallpox. He didn’t become ill.
