Then in 1798 Edward Jenner, a student of Hunter’s—Hunter had told him “Don’t think. Try.”—published his work. As a young medical student Jenner had heard a milkmaid say, “I cannot take the smallpox because I have had cowpox.” The cowpox virus resembles smallpox so closely that exposure to cowpox gives immunity to smallpox. But cowpox itself only rarely develops into a serious disease. (The virus that causes cowpox is called “vaccinia,” taking its name from vaccination.)
