In the 1840s, a Hungarian doctor noticed that when doctors performed autopsies and then helped women give birth, it led to a higher death rate among the mothers.
Dr. Ignaz Semmelweis then implemented chlorine handwashing practices throughout the hospital, and death rates dropped significantly.
According to Business Insider, at any given time, every person's hands are covered with a few 100,000 to several million germs--all of them constantly on the move.