One of the basic principles of medieval medicine was that four humors controlled the health of the human body. The balance between blood, phlegm, yellow bile, and black bile was essential for the well-being of a medieval person. Phlebotomy, or the practice of bloodletting, has been around since the Greco-Roman period. It was considered a form of surgery by medieval practitioners. They believed that organs in the body were connected by certain veins, therefore, letting the blood from a specific vein would affect a particular organ. When medieval practitioners treated sick patients, one of the first things they did was to inspect the blood. They checked the blood for its viscosity, temperature and rapidity of coagulation, and even tasted it.
Blood-letting allowed for the control of humors in a particular part of the body. It was often practiced at the end of Lent (the 40 day period before Easter, during spring). To rid themselves of the "bad humors" that accumulated in the winter, where salted meat was the most common food staple, medieval people turned to bloodletting to deal with the symptoms of scurvy, which resulted from the lack of vitamin C in their diet. When Lent began, meat was prohibited but fresh herbs and vegetables were available during the spring time. But medieval practitioners believed the bloodletting rather than the herbs and vegetables, improved the health of their patients.
The dangers of blood-letting are obvious; persons who were already sick now faced the possibility of death by infection, or the accidental cutting of nerves and arteries. Knives and other tools used in cutting were not sterilized because medieval people did not understand the concept of germs. More often than not, the result of blood-letting was either continual sickness or death of a patient. One of the most interesting aspects of medieval bloodletting was that in its origins, the practice did not start with physicians. Barbers, in addition to trimming beards, doubled as surgeons and often, a good bleeding was just was the barber ordered.