Microbes on and inside the human body

Trillions of microorganisms live in and on the human body—more individual microbes than there are people on Earth. A 2016 study estimated about 38 trillion bacteria in a representative 70-kg man, most of them in the digestive tract. That is a cell count, not a count of species; estimates vary by person and method, and bacterial cells are roughly comparable in number to the body’s own human cells.