In 2023, nothing seems more obvious than the fact that doctors should wash their hands in between patients. But this wasn’t always the case. And we owe our knowledge of this “obvious” fact to one of the most famous scientific dissidents in history: Ignaz Semmelweis. In 1846, the Vienna General Hospital had two free obstetric clinics, where underprivileged women, including prostitutes, could go to receive not only child-birthing services, but also free postpartum care for their infants. In return for the free services, the new mothers agreed to be used as subjects for the training of doctors (in the first clinic) and midwives (in the second). But giving birth in the first clinic was extremely dangerous. Almost one in 10 women who gave birth there died of puerperal fever. The rate of childbed fever was much lower in the second clinic. As word got around about this, women who knew they would be assigned to the first clinic sometimes deliberately contrived to give birth on the street, as if on the way to the hospital, so they could receive the free infant care without being subjected to the clinic’s obstetric services. The women who gave birth on the street rarely if ever contracted puerperal fever.

Semmelweis, a young physician who worked in a position comparable to what we would today call a “resident,” was one of the few people bothered by these facts. He became obsessed with figuring out why one clinic fared so much worse than the other. In 1847, one of Semmelweis’s friends was accidentally cut by a colleague’s scalpel while they were carrying out an autopsy. The friend died several days later, while exhibiting many of the same symptoms and postmortem signs as the women who died of puerperal fever. This led Semmelweis to hypothesize that the cause of the high death rate in the first clinic was the fact that medical students were carrying “cadaverous particles” from the autopsies of their anatomy lessons to their examinations of pregnant women. After experimenting with a few different cleansing agents to determine which one did the best job of removing the putrid smell of cadavers, Semmelweis proposed that medical students wash their hands with chlorinated lime between their autopsies and their examinations of patients. The mortality rate in the first clinic promptly fell by 90 percent.