Jenner acted upon a piece of rural folklore — that milkmaids who caught the mild cowpox were spared the deadly smallpox — and turned it into the first deliberate vaccination.
In 1796 he inoculated the boy James Phipps with cowpox matter, then exposed him to smallpox; the child did not sicken. From that single experiment descends the whole science of immunisation, and the only human disease yet driven from the earth.