Bloodletting, based on the humoural pathology of Hippocrates and Galen, was done in various ways for thousands of years: phlebotomy, cupping and the use of leeches. There were broad indications for applying this form of treatment. In the 19th century, P.C.A. Louis demonstrated that bloodletting was not an optimal form of treatment for pneumonia. His study was designed in accordance with the first principles of epidemiology. Epidemiology, together with the rise of Virchow's cellular pathology, contributed to the disappearance of bloodletting. The use of leeches still has a place in the medicine of the 21st century in the form of enzymes from the saliva of the leech.