Ignaz Semmelweis was a physician in Hungary in the late 1840s who worked in the maternity ward of a hospital. The mortality rate for women giving birth in his hospital at that time varied from 5% to 30% depending on the ward they were in. He was bothered by this high mortality rate and also by the significant difference in mortality rates between wards. He decided to do some research to uncover the underlying issue and, to make a long story short, he discovered that it was primarily due to the lack of physician hygiene.
Essentially, when midwives were delivering the babies in a ward, the mortality rate of the mothers was low because the midwives were relatively clean. When doctors were delivering the babies in a ward, the mortality rates of the mothers was high because the doctors did not disinfect their hands between deliveries and quite often came straight from the dissecting room where they had been practicing on corpses.
Semmelweis determined the deaths could be reduced significantly just by having the doctors, interns and midwives disinfect their hands before beginning each procedure.