Leeches are annelids comprising the subclass Hirudinea. There are freshwater, terrestrial, and marine leeches. Most species prey on small invertebrates, which they eat whole, while others feed on the blood of larger animals.
History
Leeches were formerly used in clinical bloodletting under the belief that they would cure illnesses, before medicine advanced.
During the time of Operation Snake Eater, leeches inhabited Chyornyj Prud, Tselinoyarsk. EVA's medical history also listed her as having a leech attached to her at one point, presumably from her race back to Groznyj Grad on her bike.
Behind the scenes
Leeches appear in Metal Gear Solid 3: Snake Eater, in the swamp of Chyornyj Prud. They will attempt to attach themselves onto Naked Snake in order to feed, as he swims through the water. A sucking sound can be heard to indicate that leeches attached to him. The player can prevent this by using bug juice, though its effects are only temporary.
To remove leeches from Snake, the player can access the CURE menu and burn them off with the Cigar. In reality, burning them off is inadvisable as the leeches will vomit into the wound from which they are feeding, thus increasing the chances of microbial infection from its previous hosts.
However, if you decide not to remove the leech and have it be stuck to Snake for the entire game, you'll get a special "Leech" ranking on the ranking screen.