In 1796 Edward Jenner carried out his now famous experiment on eight-year-old James Phipps. Jenner inserted pus taken from a cowpox pustule and inserted it into an incision on the boy’s arm. He was testing his theory, drawn from the folklore of the countryside, that milkmaids who suffered the mild disease of cowpox never contracted smallpox, one of the greatest killers of the period, particularly amongst children.
ref: http://www.bbc.co.uk/history/historic_figures/jenner_edward.shtml