Parasite is Pat Cadigan's novel.

Synopsis

A.D.2020. Janis is a perfectly ordinary fifteen-year-old schoolgirl until the night she greets her parents with a loaded shotgun. She counts herself unlucky that the kills only her mother. Before police can question her, Janis escapes custody with juvenile delinquent Lou. They link up with a journalist who suspects what Janis already knows: a strange new parasite is secretly displacing humans around the world. Janis has evidence that the organism took possession of her parents. But, on the run - from the infected and the still-human - her credibility and her life are forfeit...