The secret world inside you.

• Position: Microbes

The microbes in and on your body are more numerous than the stars in the Milky Way. Clump them together, and they weigh around three pounds, about as much as your brain--and, contrary to the common misconception that all microbes (the term used for organisms that are too small to be seen with the naked eye) are "germs" that cause disease, most of the ones that live in your body are vital to keeping your digestive and immune systems, as well as your brain, working properly.

"The Secret World Inside You," on view through Aug. 14 at the American Museum of Natural History, New York, uses larger-than-life models, computer interactives, videos, art installations, and a live theater to explore the rapidly evolving science that is revolutionizing how we view human health and understand the inner workings of our bodies, which are home to trillions of microbes, including bacteria, viruses, fungi, and other organisms collectively called the human microbiome. In any individual, microbial genes outnumber the genes in human DNA by more than 100 to one. Your body also contains more microbial cells than human ones--a new perspective that is changing and complicating our view of ourselves.

"Each of us is a complex ecosystem made of hundreds of species, one of which is human and the rest microbial," explains Robert DeSalle, co-curator of "Secret World" and a curator in AMNH's Division of Invertebrate Zoology and...