In July 1846, a 28-year-old Hungarian doctor named Ignaz Semmelweis started work at the Vienna General Hospital, a huge maternity hospital with two delivery clinics. The First Clinic was staffed by doctors and medical students. The Second was staffed by midwives.
The death rates were not alike. In 1846 the First Clinic lost 459 of 4,010 mothers, about 11.4%. The Second Clinic lost 105 of 3,754, about 2.8%. The killer was childbed fever, an infection that struck in the days after birth.
Semmelweis tried and ruled out explanation after explanation. Then, in 1847, his friend Jakob Kolletschka, a professor of forensic medicine, was accidentally cut with a student's scalpel during an autopsy. Kolletschka died, and Semmelweis noticed that the post-mortem showed the same kind of damage he had seen in the mothers.
The doctors in the First Clinic came straight from dissecting corpses to examining women in labour. The midwives did not do autopsies. Something on the doctors' hands was being carried into the mothers. Semmelweis could not see germs, and nobody had yet shown that they cause disease. He called it "cadaverous particles".
In mid-May 1847 he ordered everyone to wash in a solution of chlorinated lime before touching a patient. In April the First Clinic's death rate had been 18.3%. In June it was 2.2%, then 1.2% in July and 1.9% in August, about the level of the midwives' ward.
Try it below. Each dot is one mother. Set how many doctors wash their hands, then run 200 births. The risk is a simple blend between the April rate (nobody washes) and a roughly 2% rate (everybody washes). It is a model of the idea, not a replay of the real wards.
Green dots recovered, red dots died. Run it a few times: chance makes each batch a little different, which is why Semmelweis needed months of figures, not one good week.
Semmelweis had the data and a rule that worked, but no mechanism, and his claim implied that doctors were killing their patients. Many colleagues resisted it. He published his main work, The Etiology, Concept and Prophylaxis of Childbed Fever, in 1861. In July 1865 he was committed to an asylum, and he died there on 13 August 1865, aged 47, of an infected wound on his hand.
His findings were accepted widely only years later, after Louis Pasteur confirmed the germ theory of disease and Joseph Lister brought hygienic methods into surgery.
A pattern in numbers can be real before anyone knows why. Semmelweis found the cause by comparing two wards, changing one thing, and counting. The washing worked long before germs were understood.