Your body is teeming with tens of trillions of microbes. It's an entire world, a colony full of life. In other words, you contain multitudes. These microscopic companions sculpt our organs, protect us from diseases, guide our behaviour, and bombard us with their genes. They also hold the key to understanding all life on Earth. Ed Yong opens our eyes and invites us to marvel at ourselves and other animals in a new light, less as individuals and more as thriving ecosystems. We learn the invisible and wondrous science behind the corals that construct mighty reefs and the squid that create their own light shows. We see how bacteria can alter our response to cancer-fighting drugs, tune our immune system, influence our evolution, and even modify our genetic make-up. And we meet the scientists who are manipulating these microscopic partners to our advantage.