From New York Times bestselling author Mira Grant comes a vision of a decade in the future, where humanity thrives in the absence of sickness and disease. We owe our good health to a humble parasite — a genetically engineered tapeworm developed by the pioneering SymboGen Corporation. When implanted, the Intestinal Bodyguard worm protects us from illness, boosts our immune system — even secretes designer drugs. It's been successful beyond the scientists' wildest dreams. Now, years on, almost every human being has a SymboGen tapeworm living within them. But these parasites are getting restless. They want their own lives . . . and will do anything to get them.