I ban violakat for not knowing either.
Parasite Eve was one of the original Playstation launch titles, and generally seen as one of the best video games of all time.
Parasite Eve is about the mitochondria in humans' cells evolving sentience of their own and taking over their human hosts' bodies. They can release all their energy at once and make the humans burst into flame, or they can convert the humans' bodies into usable biomatter with which to complete their missions.
Aya is a New York City cop who has mitochondria that evolved in a different way. Hers HELPS the host. Basically she develops super-powers, some of them really awesome. And of course her different mitochondria keeps her immune to the effects of the "bad guy" mitochondria.