IN THE MID-1840s Hungarian physician Ignaz Semmelweis saw with alarm that 15 percent of new mothers in his Vienna General Hospital were dying of an illness called puerperal fever. Semmelweis was desperate to prevent the illnesses, but he didn't know how. As he pondered the problem, he learned that his friend, forensic pathologist Jakob Kolletschka, had died from what sounded like the same illness. It happened only a few days after a student accidentally pricked Kolletschka with a scalpel that had been used to dissect a cadaver.