Today we study the brain with painstaking detail. But we largely ignore what might prove to be the most important part of our body: the trillions of microbes that live inside us, especially in our gut. Increasingly, it looks like the harmless bacteria we each have - or lack - may hold the key to our most vexing health issues, including obesity, allergies, asthma and digestive problems. The makeup of this internal ecosystem varies wildly from person to person. Different people - even those in the same ethnic group, geographic region and immediate family - may have up to a 90% difference in the contents of our gut. (Compare this with our DNA, where all humans, worldwide, are 99.99% the same.) Evidence shows that small changes in the microbes present (altered by antibiotics, diet, geographic region, and so on) may affect weight, likelihood of disease and even psychological factors like risk-taking behaviour. Knight and Buhler explore the previously unseen world inside our bodies, in dynamic, accessible prose geared towards a general audience. With a practical eye towards deeper knowledge and better decisions, the book includes a detailed tour of our "micro-biome" as well as an exploration of the known effects of antibiotics, probiotics, diet choices, birth method, and access to livestock on our children's lifelong health. The book will guide readers on how to learn about their own "microbiome" and take steps towards understanding and improving their health, using the latest research as their guide.