Edward Jenner, British physician, c 1800.

UNITED KINGDOM - OCTOBER 22: Lithograph by Leon Noel, based on Vignero's version of a drawing by J R Smith. Jenner (1749-1823), an English doctor and pupil of John Hunter, introduced fluid from a cowpox sore through a person's skin, in order to protect them against smallpox, a similar but more dangerous disease. He called his method 'vaccination' from 'vacca', the Latin word for cow. , published as 'Inquiry into the Cause and Effects of Variolae Vaccinae [cow-pox]' in 1798, although he did not understand why the method worked, his scientific investigations showed that using fluid from a human with cowpox was safer than variolation; inoculating non-infected people with fluid from pustules of smallpox. The 1853 Vaccination Act heralded an era of compulsory vaccination against smallpox. (Photo by SSPL/Getty Images)