Discovery of vaccination

Edward Jenner, a British scientist, discovered that vaccinations could be a way of preventing disease. Jenner heard anecdotally that dairymaids had developed resistance to smallpox after suffering from cowpox, a much milder disease.

On 14 May 1796, Jenner conducted an experiment where he deliberately infected a child with cowpox.

In July, shortly after the child's recovery from cowpox, Jenner injected him with matter from fresh smallpox lesions. The child did not contract the disease, leading Jenner to conclude that the child had been made immune through the inoculation.

Source(s)

Riedel S.
Edward Jenner and the history of smallpox and vaccination.
Proc (Bayl Univ Med Cent).
2005; 18(1): 21–25.