These tiny organisms can only live inside another animal, and as McAuliffe reveals, they have many evolutionary motives for manipulating their host’s behavior. Far more often than appreciated, these puppeteers orchestrate the interplay between predator and prey. With astonishing precision, parasites can coax rats to approach cats, spiders to transform the patterns of their webs, and fish to draw the attention of birds that then swoop down to feast on them. We humans are hardly immune to the profound influence of parasites. Organisms we pick up from our own pets are strongly suspected of changing our personality traits and contributing to recklessness, impulsivity—even suicide. Microbes in our gut affect our emotions and the very wiring of our brains. Germs that cause colds and flu may alter our behavior even before symptoms become apparent. Parasites influence our species on the cultural level too. As McAuliffe documents, a subconscious fear of contagion impacts virtually every aspect of our lives, from our sexual attractions and social circles to our morals and political views. Drawing on a huge body of research, she argues that our dread of contamination is an evolved defense against parasites—and a double-edged sword. The horror and revulsion we feel when we come in contact with people who appear diseased or dirty helped pave the way for civilization, but may also be the basis for major divisions in societies that persist to this day. In the tradition of Jared Diamond’s Guns, Germs and Steel and Neil Shubin’s Your Inner Fish, This Is Your Brain on Parasites is both a journey into cutting-edge science and a revelatory examination of what it means to be human.