More than 100 trillion microbes live in the dark and nearly oxygen-free world of the human gut—about the same number of all the human cells in the body, if you include the human red blood cells in this comparison. This means that only 10 percent of the cells in or on a human being are actually human. (If you include the body’s red blood cells, this number may be closer to 50 percent). If you put all your gut microbes together and shaped them into an organ, it would weigh between 2 and 6 pounds—on par with the brain, which weighs in at 2.6 pounds.
