Welcome to Humanville, a lively metropolis of over a hundred trillion bacteriaa hundred trillion bacteriaa hundred trillion bacteria living in and around your body. Like many cities, most of Humanville’s denizens live in its centre – the bowels – although a sizeable population have set up shop in the surrounding suburbs of the skin. The residents of Humanville, collectively known as the microbiome, are model citizens. They’re the unseen force that processes much of the city’s food supply, regulates its defences against invaders, and keeps it working like a healthy, well-oiled machine.

Humanville isn’t alone. It is one of many similar bacterial conurbations, each thriving in the body of a different animal. Those that live in