Genetics and lifestyle are thought to be the two most important determinants of good health. But that is not the whole story. We have a second genome, our gut bacteria, that sets the dial on our bodies. Unlike our DNA, we can influence the gut bacteria, or microbiota, to optimize all aspects of our health. In The Good Gut, noted Stanford researchers Justin and Erica Sonnenburg, who are doing cutting-edge research on the microbiota, investigate how the trillions of microbes that reside in our gastrointestinal tract help define us, affecting everything from our immune response to our weight, allergic reactions, aging and emotions; how they are under threat from the Western diet, our antibiotics, and our sterilized environment; and how we can nurture our individual microbiota. This is urgent news. The recent change in our gut microbiota is linked to the alarming increase in obesity and autoimmune diseases. Our intestinal microbiota play an important role in the prevalence of predominantly Western afflictions, such as cancer, diabetes, allergies, asthma, autism, and inflammatory bowel diseases. These gut bacteria are facing a mass extinction, and the health consequences are dire. The average American has 1,200 different types of bacteria residing in his or her gut. That may seem like a lot until you consider that the average Amerindian living in the Amazon has 1,600 species and is much less likely to develop Western maladies. How can we keep our microbiota off the endangered species list? How can we strengthen the community that inhabits our gut and thereby improve our own health? Your prescription for gut health is unique to you, and it changes as you age. The Good Gut offers a new plan for health that focuses on how to nourish your microbiota, including recipes and a menu plan. Drs. Sonnenburg look at safe alternatives to antibiotics; dietary and lifestyle choices to encourage microbial health; the management of the aging microbiota; and the nourishment of your own individual microbiome. The proper understanding and care of our gut may be the most important health choice we can make.