Your body contains trillions of bacteria that live mostly inside the digestive system.

The longer version

Trillions of microbes live along the digestive tract, with by far the greatest numbers in the large intestine, where movement is slow and conditions are stable. Most of them are bacteria that break down fibre and other material the body's own enzymes cannot handle, producing short-chain fatty acids and certain vitamins in the process.

They also occupy space and consume resources, which makes it harder for arriving pathogens to establish themselves. The immune system constantly samples this population and learns to tolerate the usual residents. Composition differs noticeably from person to person, shaped by diet, environment and early life. Roughly as many of these cells live in the body as there are human cells, by most estimates.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.