Although we are, by number at least, 90% bacteria, only recently have scientists with the Human Microbiome Project have just begun figuring out who those bugs are.
[The] trillions of microbes that make their homes in and on us do an excellent job keeping us healthy (crowding out harmful microbes) and sated (breaking down a lot of the food we ingest).
Now that disturbances in this rich microbiome community have been linked to weight gain, inflammatory bowel disease, vaginal infections and risk for infection with harmful microbes (such as methicillin-resistant Staphylococcus aureus, or MRSA), the importance of understanding what makes up a “healthy” microbiome has become even more apparent.
— Harmon (2012): Body Count: Taking Stock of All the Bugs That Call Humans Home in Scientific American.