Millions, even trillions, of bacteria use the human body as home base, mostly living peacefully and without making us sick.
For the first time, microbiologists have mapped and documented the bacteria and other microbes that live on or in healthy people.
More than 200 scientists were involved in this massive undertaking called the "Human Microbiome Project".
Bacteria are essential to digest food and metabolize vitamins, even fight off bad bacteria. However, scientists were surprised to find disease-causing bacteria co-existing with good bacteria, even on healthy people.
The next step will be to determine why some people get sick while others remain healthy.