The understanding that specific infectious diseases are caused by specific microorganisms — bacteria, viruses, fungi, and parasites — that invade the body from outside. Germ theory replaced miasma theory, the ancient and medieval belief that disease arose from bad air and environmental corruption, and in doing so transformed medicine from a largely ineffective art into an increasingly precise science.
In 1847, Ignaz Semmelweis noticed that women in Vienna's First Obstetrical Clinic were dying of childbed fever at three times the rate of women in the Second Clinic. The difference: the First Clinic was staffed by medical students who came directly from performing autopsies. Semmelweis concluded that the students were transmitting something from the cadavers to the patients. He mandated handwashing with chlorinated lime solution. Death rates dropped from 10% to 1%. The medical establishment rejected his findings. He was dismissed, ridiculed, and eventually committed to an asylum, where he died — probably of the same infection he had spent his career fighting. Germ theory was not a comfortable idea. It implicated doctors themselves in the deaths of their patients.