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.

The news gave Semmelweis pause. Medical students at his hospital would routinely go right from the morgue to the maternity ward without ever washing their hands. Were they carrying an infection to the mothers? Was that why they were dying? Could hand washing help?

To test his dirty-hands hypothesis, Semmelweis made his students wash their hands in a mixture of water and chlorine (soap and water did not eliminate the cadaver smell). Fevers in the maternity ward quickly dropped by 10 percent. Hand washing became standard procedure at Semmelweis's hospital.

It took 40 years for the policy to take hold widely. Even today hospital workers don't follow it as consistently as they should. According to an ongoing study from the Maryland Health Quality and Cost Council, 90 percent of staff wash their hands when someone is looking, but only 40 percent do when alone.