Parasite Eve (1997)

Toshiaki Nagashima is a biologist who is doing major research on mitochondria. When his beautiful young wife is tragically involved in a car accident which leaves her brain dead, in desperation he steals her liver from her body in order to receive the mitochondria from it to resurrect his wife from the dead. The killer mitochondria takes the form of his assistant who then uses the biologist for the host of a terrifying new species that is threatening to take over the world.