Almost every animal will at some time or other become the home of a parasite. Not only are parasites the most sucessful life-forms on Earth, they triggered the development of sex, shape, ecosystems, and have driven the engine of evolution. Carl Zimmer describes the frightening and amazing ingenuity these commando invaders use to devour their hosts from the inside and control their behaviour. 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. Single-celled Toxoplama gondi has an even more insidious role, for it can invade the human brain and cause personality changes, making its host less afraid and more prone to danger and a violent end - so that, in the carnage, it will be able to move on to another host. Finally, 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.