Every human body carries a secret cargo: a huge population of micro-organisms that lives in your mouth and nose, on your skin, in your guts. This is the human microbiome, and it has in the last few years become biologists most thrilling research interest. Vastly outnumbering human cells, our invisible passengers are vital to life. They help us digest food, make essential nutrients, and combat disease. They help regulate allergies and inflammation via the immune system. They influence bowel disease, heart disease and cancer. They may even have a role in developing behaviour. We are not just isolated humans, but superorganisms. And that calls into question our obsession with disinfection and sterilisation, and our antibiotic addiction, which is like managing a complex ecosystem with napalm. Acclaimed science communicator and editor Jon Turney presents the first book-length account of a new realm of human biology that will make you think again about this machine we call ourselves.