Wikipedia: "Yearly Mortality Rates"
Study this graph, which was generated from the data published by Semmelweis in 1861. In the Wien maternity clinic, pathological anatomy was permitted; thus, physicians and medical students could carry pathogens from cadavers to patients. The Dublin maternity hospital had no medical pathology. Note the death rate drop after the introduction of the chlorine handwash. Wikimedia Commons user Power.corrupts constructed this graph using Semmelweis' data.