Minutes after you were born, bacteria moved in. Since then, their populations have exploded, diversified and spread on your skin and eyes and in your mouth and gut, not to mention other places. These bacterial cells far outnumber your own cells.

Some bacteria on your skin can cause infections, including infections of antibiotic-resistant bacteria like methicillin-resistant Staphylococcus aureus (commonly known as MRSA). Preventing such illnesses is the reason for those restroom signs about proper hand-washing.

But most bacteria on your skin are harmless, and some are actually very helpful. They ward off more dangerous bacteria, aid wound healing and shelter us from certain skin infections. For instance, Staphylococcus epidermidis protects us by taking up space that a more harmful bacterium would otherwise occupy.

Understanding how and why bacteria colonize particular places on the body could point to ways of treating skin and other conditions.