Possession

At some point of the story, this character gets possessed by another being.

A preternatural entity takes residence in this person's body. The degree of control may vary, but the possessing entity is both foreign and not a naturally occurring phenomenon. Furthermore, the preternatural entity may take residence in an uninhabited vessel (such as a corpse).

Possession only covers being taken over by a metaphysical entity (spirit, ghost, demon, deity, etc.). In case of physical infections by a biological organism, just use Parasitism instead.