The body normally contains several hundred different species of bacteria but many trillions of individual bacteria.

Most of these bacteria reside in the following places:

• On the skin and teeth

• In the spaces between teeth and gums

• In the mucous membranes that line the nose and nasal passages, throat, intestine, and vagina

The species differ at each site, reflecting the different environment at each site.

Many of them are anaerobes—that is, they do not require oxygen to live and grow.

Usually, these anaerobes do not cause disease. Many have useful functions, such as helping break down food in the intestine.

However, anaerobic bacteria can cause disease if the mucous membranes are damaged. Then, bacteria can enter tissues that are usually off-limits to them and that have no defenses against them. The bacteria may infect nearby structures (such as the sinuses, middle ear, lungs, brain, abdomen, pelvis, and skin) or enter the bloodstream and spread.