For many years nineteenth century English physician Edward Jenner worked to find a cure for smallpox. After studying many cases, he reached an impasse in his thinking. Then he reversed his perception of the problem. Instead of focusing on people who had smallpox, he switched his attention to people who never had smallpox. He found that dairymaids rarely got the disease. It turned out that most dairymaids had had cowpox, a similar but usually nonfatal affliction. Cowpox had served to “vaccinate” its victims against the more dangerous small pox. This led to Jenner’s concept of “vaccinating” people.