Leech

Leeches are annelids comprising of the subclass Hirudinea. There are freshwater, terrestrial, and marine leeches. They are parasitic creatures that feed by sucking on human blood, although the majority simply eat small invertebrates whole.

History
Leeches were formerly used to thin blood under the belief that they would cure illnesses, before medicine advanced.

During Operation Snake Eater, leeches were living in Chyornyj Prud and also attached themselves to various soldiers bodies to drain their blood unless they wore bug juice.

Behind the scenes
If the leeches attached themselves to Snake, the player can go into the CURE menu and place a cigar onto them to burn them off. In reality, burning them off with a cigar is inadvisable as leeches, when prodded with a burning object, will vomit into the wound that they are attempting to drain the blood from, thus increasing the chances for infection.