From New York Times bestselling author Mira Grant comes a high-concept near-future thriller. A decade in the future, 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 its host from illness, boosts the immune system—and even secretes designer drugs. It's been successful beyond the scientists' wildest dreams. Now, years on, almost all human beings have a SymboGen tapeworm living within them. But these parasites are getting restless. They want their own lives—and will do anything to get them.