In the 1840s, in a Vienna maternity hospital, women in the ward run by doctors were dying of fever far more often than women in the ward run by midwives. Semmelweis noticed that the doctors came straight from doing autopsies.
He made them wash their hands in chlorinated lime. Deaths dropped sharply. He had no theory of germs to explain why, only the numbers.
The medical establishment didn't accept it, partly because it suggested they were killing their patients. He got angrier and more erratic, lost his position, and died in an asylum at 47. Germ theory proved him right a few years later.