Bleeding was a popular treatment
for anyone who believed in the four humours
, which means anyone from Greek times right up to the 18th/19th century. Because Galen
taught that blood was being constantly created in the liver
and pumped around the body, it was thought an excess could sometimes build up. Bleeding involved cutting the patient to let some of this excess blood out.
As well as been used reactively to treat diseases, many people were bled regulary proactively to stave off disease.
Even when William Harvey, around 1615, proved that blood was not constantly been created, but was instead passed around the body again and again, the practice of bleeding continued.