Now scientists have mapped which critters normally live in or on us and where. They've calculated that healthy people share their bodies with more than 10,000 species of microbes.
Don't say "eeew" yet. Many of these organisms work to keep humans healthy. Results reported Wednesday from the government's Human Microbiome Project define what's normal in this mysterious netherworld.
Next they'll explore what doctors really want to know: Why do bad bugs harm some people and not others?