Parasyte: The Maxim

Shinichi Izumi is a normal high school boy whose right hand has become infected with an alien parasite that names itself "Migi" ("right"). Migi is the first parasite to develop a symbiotic relationship with its host, as he and Shinichi slowly develop a grudging friendship. Migi isn't the only Parasite on earth, however, and as cases of Parasites killing humans begin to emerge, humans seek to kill off Parasites. Shinichi and Migi find themselves caught in between these two sides of the struggle over planet earth.

Synopsis from Anime News Network