The next step is to explore what doctors really want to know: Why do the bad bugs harm some people and not others? What changes a person's microbial zoo that puts them at risk for diseases ranging from infections to irritable bowel syndrome to psoriasis?

Already the findings are reshaping scientists' views of how people stay healthy, or not.

"This is a whole new way of looking at human biology and human disease, and it's awe-inspiring," said Dr. Phillip Tarr of Washington University at St. Louis, one of the lead researchers in the $173 million project, funded by the National Institutes of Health.

"These bacteria are not passengers," Tarr stressed. "They are metabolically active. As a community, we now have to reckon with them like we have to reckon with the ecosystem in a forest or a body of water."

And like environmental ecosystems, your microbial make-up varies by body part. Consider your underarm a rainforest.