A disease could be caused by old and sick blood, or by an un-balance of the liquids of the body. Some blood was then let out. A knowledgeable woman, a blood-letter, opened a vein with a special knife. The blood was let near the sick spot. By looking at the colour of the blood and by measuring the contents, it was possible to tell how severe the disease was. It was thought that if the sick blood was let out, the disease would disappear. Sometimes one used leeches. They were put on the skin to suck out old blood.
If one let one’s blood four times a year, one was expected to stay healthy. After the bloodletting, one had to eat something good and relax. In some monasteries, there was a rule against the monks letting their blood more than four times a year. The bloodletting meant that they would be too weak to do their work properly.