Healthy people live in harmony with most microorganisms that establish themselves on or in (colonize) the body. The microorganisms that usually occupy a particular body site are called the resident flora. Cells of the resident flora outnumber a person's own cells 10 to 1. Microorganisms that colonize people for hours to weeks but do not establish themselves permanently are called transient flora.

The resident flora at each site includes several different types of microorganisms. Some sites are normally colonized by several hundred different types of microorganisms. Environmental factors—such as diet, sanitary conditions, air pollution, and hygienic habits—influence what species make up a person's resident flora. If transiently disturbed (for example, by washing or use of antibiotics), the resident flora usually promptly reestablishes itself.

Rather than causing disease, the resident flora often protects the body against disease-causing organisms. However, under certain conditions, microorganisms that are part of a person's resident flora may cause disease. Such conditions include the use of antibiotics and a weakened immune system (as occurs in people with AIDS or cancer, people taking corticosteroids, and those receiving chemotherapy). When antibiotics used to treat an infection kill a large proportion of certain types of bacteria of the resident flora, other resident bacteria or fungi can grow unchecked. For example, if women take antibiotics for a bladder infection, the antibiotics kill some of the resident flora, allowing yeast in the vagina to multiply and cause a vaginal yeast infection.

Last full review/revision October 2012 by Allan R. Tunkel, MD, PhD