English physician who investigated the old wives' tale that anyone who caught cowpox would be immune to smallpox. In his experiment, he took the blister fluid of Sarah Nelmes, a milkmaid with cowpox, and injecting it into a boy named James Phipps. After the boy recovered from cowpox, Jenner injected him with smallpox. Luckily, the experiment proved a success, and the boy proved to be immune from smallpox. After repeating the demonstration, Jenner published his results and vaccination (from the Latin words "vacca" for cow and "vaccinia" for cowpox) quickly became a widespread practice.