For centuries, parasites have lived in nightmares, horror stories, and in the darkest shadows of science. Now award-winning writer Carl Zimmer takes us on a fantastic voyage into the secret parasite universe we actually live in but haven’t recognized. He reveals not only that parasites are the most successful life-forms on Earth, but that they triggered the development of sex, shape ecosystems, and have driven the engine of evolution. In mapping the parasite universe, Zimmer makes the astonishing observation that most species are parasites, and that almost every animal, including humans, will at one time or another become the home of a parasite. Zimmer shows how highly evolved parasites are and describes the frightening and amazing ingenuity these commando invaders use to devour their hosts from the inside and control their behavior. The sinister Sacculina carcini makes its home in an unlucky crab and proceeds to eat everything but what the crab needs to put food in its mouth, which Sacculina then consumes. When Sacculina finally reproduces, it places its young precisely where the crab would nurture its own progeny, and then has the crab nurture the foster family members. Single-celled Toxoplasma gondi has an even more insidious role, for it can invade the human brain. There it makes men distrustful and less willing to submit to social mores. Women become more outgoing and warm-hearted. Why would a parasite cause these particular personality changes? It seems Toxoplasma wants its host to be less afraid, to be more prone to danger and a violent end — so that, in the carnage, it will be able to move on to another host. From the steamy jungles of Costa Rica to the fetid parasite heaven of rebel-held southern Sudan, Zimmer tracks the genius of parasitic life and its impact on humanity. We hosts have developed remarkable defenses against the indomitable parasite: our mighty immune system, our culturally enforced habit of keeping clean, and, perhaps most intriguingly, sex. But this is not merely a book about the evil power of parasitism and how we must defend against it. On the contrary, Zimmer concludes that humankind itself is a new kind of parasite, one that preys on the entire Earth. If we are to achieve the sophistication of the parasites on display here in vivid detail, if we are to promote the flourishing of life in all its diversity as they do, we must learn the ways nature lives with itself, the laws of Parasite Rex.