Woodcut of a woman standing at a table has placed a leech on her left forearm; on the table is a large jar containing leeches. Bloodletting was based on an ancient system of medicine in which blood and other bodily fluid were considered to be "humors" the proper balance of which maintained health, but it often weakened sick patients. It was the most common medical practice performed by doctors from antiquity up to the late 19th century, a time span of almost 2,000 years. Leeches could also be used. The withdrawal of so much blood as to induce syncope (fainting) was considered beneficial, and many sessions would only end when the patient began to swoon.