Sometime during the late 1760s whilst serving his apprenticeship as a surgeon/apothecary Edward Jenner learned of the story, common in rural areas, that dairy workers would never have the often-fatal or disfiguring disease smallpox, because they had already had cowpox, which has a very mild effect in humans.
The twentieth century saw the introduction of several successful vaccines, including those against diphtheria, measles, mumps, and rubella.