They live on your skin, up your nose, in your gut — enough bacteria, fungi and other microbes that collected together could weigh, amazingly, a few pounds.
It turns out that nearly everybody harbors low levels of some harmful types of bacteria, pathogens that are known for causing specific infections. But when a person is healthy — like the 242 U.S. adults who volunteered to be tested for the project — those bugs simply quietly coexist with benign or helpful microbes, perhaps kept in check by them.
And like environmental ecosystems, your microbial makeup varies widely by body part. Your skin could be like a rainforest, your intestines teeming with different species like an ocean.