a usually black blood-sucking annelid of the class Hirudinea, commonly found in fresh water or damp tropical environments. They have good infrared sensors and highly effective "suckers" or attachment discs. Although revolting to the uninitiated, they do no permanent harm and were commonly used by medieval doctors to bleed patients when this was thought to be useful; hence the transference of the name. They have found recent application in surgical reattachment of severed fingers, etc., where arteries have been reconnected but veins have not, by providing enough "circulation" for angiogenesis to recreate a new veinule network.

Removal should be with sprinkled salt, or if you know someone who smokes, cigaret burns are equally effective. Leeches inject anti-clotting agent, so leech bites will bleed much longer than expected, and must be protected against secondary infection.