Healthy people live in harmony with most of the microorganisms that establish themselves on or in (colonize) nonsterile parts of the body, such as the skin, nose, mouth, throat, large intestine, and vagina. 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, antibiotic use, sanitary conditions, air pollution, and hygienic habits, influence what species make up a person’s resident flora. If temporarily disturbed (for example, by washing the skin or using 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
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.
Injury or sometimes surgery can allow resident flora to enter areas that are not supposed to have bacteria and cause infection. For example, a cut on the skin can allow resident skin flora to cause an infection under the skin. Surgery on the large intestine sometimes allows the resident flora in the intestine to spill into sterile areas in the abdomen and cause very serious infection.
(See also Overview of Infectious Disease.)