A question posed at Metafilter
via an article in the New York Times.

Of the trillions and trillions of cells in a typical human body — at least 10 times as many cells in a single individual as there are stars in the Milky Way — only about 1 in 10 is human. The other 90 percent are microbial. These microbes — a term that encompasses all forms of microscopic organisms, including bacteria, fungi, protozoa and a form of life called archaea — exist everywhere. New evidence suggests microbes in our bodies can determine how efficiently we process food and affect our hunger centers.