Across the planet, a battle is raging, between parasites and their hosts. There is hardly an organism on this planet that doesn't have parasites that are trying to get at it. Some parasites destroy their hosts with terrifying speed, while others have a more cunning strategy: they keep their host alive for as long as possible. They lurk inside the body and steal their host's nutrients. Some of these parasites are almost impossible to kill. They can evade the immune systems. They can hide in the darkest recesses of the body, and they can survive the most powerful drugs. Once they strike, their victims are condemned to a life sentence of living with the enemy inside them. In Virginia a young woman is attacked by an invincible brain-eating monster that threatens to take her life. A Midwestern farmer does battle with a beast that cannot be killed and a New Yorker is infected with the deadliest parasite on the planet. Three parasites that are almost impossible to destroy; three victims, fighting for their lives. If doctors cannot cure them their only hope of survival is by living with the enemy inside them.