In the 1840s the maternity clinic of the Vienna General Hospital held a terrifying puzzle. In one of its two obstetric wards — the one staffed by doctors and medical students — the rate of mothers dying from childbed fever (a deadly post-partum infection) was several times that of the other ward, staffed by midwives. Women were horrified to learn which ward they would fall into.

The Hungarian physician Ignaz Semmelweis (1818–1865) chased this difference like a detective. The turning point came when a colleague, cut by a student's scalpel during an autopsy, died with the same symptoms as childbed fever. Semmelweis made the connection: the doctors performed autopsies in the morning, then delivered babies without washing their hands. Some invisible "thing" carried from the corpses to their hands must be transmitting the infection to the mothers. The midwives did no autopsies — there lay the difference.

In 1847 Semmelweis set a simple rule: before entering a delivery, hands were to be washed in a solution of chlorinated lime. The result was dramatic; within months the death rate in the doctors' ward fell to that of the midwives', in some months near zero. He documented this with meticulous statistics. There was as yet no concept of bacteria; Semmelweis did not know what was transmitted, but he had shown that something was, and how to stop it.

The tragedy is that his age rejected him. His colleagues met the idea that their own hands caused the deaths as an insult; Semmelweis was ostracised, gradually broke down, and died in an asylum, probably of an infection. Only years later did the germ theory of Pasteur and Lister give a name to what he had seen in his numbers. Semmelweis is today counted a pioneer of evidence-based medicine and hospital infection control; his story tells how hard it is for even a correct finding to be accepted without an explanation.