Every human body carries a secret cargo: a huge population of microorganisms living in the mouth, on the skin, in the gut. They help digest our food. They make essential vitamins. They break down toxins and metabolise drugs. They exert an invisible influence on our hormones, our immune systems, perhaps even our brains. This is the human microbiome - a living, shifting system of previously unimagined importance and complexity. In this first book-length account of this new realm of human biology, award-winning science writer Jon Turney explores the microbiome in detail, charting its birth and development, investigating how it works, and assessing its many implications for our health, including its potential to shed new light on conditions such as bowel diseases, cancer, allergies and asthma. He considers the potential impacts of our modern disinfectant and antibiotic obsessions, and ponders a future of designer microbiomes and mood-altering probiotics. This book will make you think again about your relationship with your body, your habits - even your sense of who and what you are - as it reveals what it means to be a 21st century superorganism.