Edward Jenner
On 14 May 1796, Jenner tested his hypothesis on an eight-year-old boy, the son of his gardener. He scraped pus from cowpox blisters on the hands of a milkmaid. Jenner inoculated Phipps in both arms that day, subsequently producing in Phipps a fever and some uneasiness, but no full-blown infection. Later, he injected the boy with a mild form of smallpox and he showed no sign of infection. Jenner successfully tested his hypothesis on 23 more subjects.