You are just 10% human. For every one of the cells that make up the vessel that you call your body, there are nine impostor cells hitching a ride. You are not just flesh and blood, muscle and bone, brain and skin, but also bacteria and fungi. Over your lifetime, you will carry the equivalent weight of five African elephants in microbes. You are not an individual but a colony.

Until recently, we had thought our microbes hardly mattered, but science is revealing a different story, one in which microbes run our bodies and becoming a healthy human is impossible without them.

In this riveting, shocking, and beautifully written book, biologist Alanna Collen draws on the latest scientific research to show how our personal colony of microbes influences our weight, our immune system, our mental health, and even our choice of partner. She argues that so many of our modern diseases—obesity, autism, mental illness, digestive disorders, allergies, autoimmunity afflictions, and even cancer—have their root in our failure to cherish our most fundamental and enduring relationship: that with our personal colony of microbes.

Many of the questions about modern diseases left unanswered by the Human Genome Project are illuminated by this new science. And the good news is that unlike our human cells, we can change our microbes for the better. Collen's book is a revelatory and indispensable guide. It is science writing at its most relevant: life—and your body—will never seem the same again.