Leeches are worms that live in freshwater environments. The majority feed on blood using two suckers, one at each end of their bodies. In medicine, leeches have been used for bloodletting for thousands of years. This is a process whereby a small amount of blood is removed from the body in the belief that blood is a ‘humour’ that needs to be kept in balance. Removing ‘excess’ blood was thought to bring the body back into balance and so cure a variety of symptoms.