Ans:In 1796, a British physician, Edward Jenner, infected a young boy with cowpox, by injecting pus cells. After the boy had recovered from cowpox, Jenner injected the pus cells from a smallpox patient into him. the boy did not get smallpox. So it became clear that intentional infection with cowpox protected people from smallpox. This method was named "vaccination" and the substance used to vaccinate was called a "vaccine". 1