People usually think of microbes as the root cause of diseases, while also associating them with germs and dirt. However, in fact, microbes are indispensable to living organisms on earth. Humans and microbes both cooperate and coexist. In the human body, microbes play multiple important roles; such as shaping the organs, modulating the immune system, and digesting food. This book tells the story of the symbiotic relationship between organisms, and the roles that microbes play in all spheres of life. It will change the way we look at our bodies, and the world.