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Normal flora is the term used to describe the various bacteria and fungi that are permanent residents of certain body sites, especially the skin, oropharynx, colon, and vagina (Tables 6–1 and 6–2). Viruses and parasites (protozoa and helminths), which are the other major groups of microorganisms, are usually not considered members of the normal flora, although they can be present in asymptomatic individuals. The normal flora organisms are often referred to as commensals. Commensals are organisms that derive benefit from another host but do not damage that host. The term human microbiome is often used to describe the normal flora (see later).

TABLE 6–1Summary of the Members of Normal Flora and Their Anatomic Locations
TABLE 6–2Medically Important Members of the Normal Flora

The members of the normal flora vary in both number and kind from one site to another. Although the normal flora extensively populates many areas of the body, the internal organs usually are sterile. Areas such as the central nervous system, blood, lower bronchi and alveoli, liver, spleen, kidneys, and bladder are free ...

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