Resident Flora
A healthy person lives in harmony with most microorganisms that establish themselves on (colonize) the body. The microorganisms that usually occupy a particular body site are called the resident 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. Rather than causing disease, the resident flora often protects the body against disease-causing organisms. If disturbed, the resident flora promptly reestablishes itself. Microorganisms that colonize the host for hours to weeks but do not establish themselves permanently are called transient flora.
Environmental factors--such as diet, sanitary conditions, air pollution, and hygienic habits--influence what species make up a person's resident flora. 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 weakening of the immune system (as occurs in people with AIDS and cancer, people taking corticosteroids, and those receiving chemotherapy). When antibiotics used to treat an infection kill a large proportion of the resident flora of the skin, vagina, or intestine, other resident bacteria or fungi can grow without being held in check. An example is a vaginal yeast infection occurring in a woman taking antibiotics for a bladder infection.
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