Edward Jenner wanted to trial his hypothesis based on countryside folklore, that milk maids never contracted smallpox.
In 1796, Jenner took experiments on 8 year old James Phillip. He takes pus from a cowpox from a cowpox pustule, makes a cut in James's  arm and inserts the liquid.
James contracted cowpox, and then recovers, only suffering a light fever for a few days.
A few weeks after recovery,  Jenner gives James smallpox by using the same previous method.
James remains healthy and does not contract smallpox.
Jenner names  this procedure "Vaccine", coming from the Latin word for cow . "Vacca".