Leeches are freshwater worms that feed on the blood of other animals. They were used medicinally for thousands of years when bloodletting was mainstream practice. In Ancient Greece, bloodletting (which is simply the removal of blood) was used to balance the four “humors” within a person: blood, phlegm, black bile, and yellow bile. Making their comeback in the 1980s, leeches have, once again, become a widely accepted tool in medicine.
The salivary glands of leeches contain a blood anticoagulant called Hirudin that earns them a hot seat in the operating room. The leeches are placed on a patient’s surgical flap to suck up any excess coagulated blood. This prevents the tissue from dying. Hirudotherapy is now accepted practice in many countries, including the US. Medicinal leeches are grown in captivity to remove any risk of infection and can be purchased online for as little as $8 each.
Wikipedia: Medicinal Use of Leeches