Infection

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Infection, injurious contamination of the body or part of the body by pathogenic agents, such as fungi (see Fungus Infections), bacteria, protozoa, rickettsiae, or viruses, or by the toxins that these agents may produce. An infection may be local and confined to one area or generalized and spread throughout the body. Once the infectious agent enters the host and begins to proliferate, the defense mechanisms of the body react to the infection, producing the characteristic symptoms of pain, swelling, reddening at the site of infection, functional disorders, rise in body temperature and pulse rates, and an increase in the number of white cells (see Immune System).

Infectious agents may enter the body of the host by several routes. The most common route is through the respiratory passages, urinary tract, and gastrointestinal tract, but other common entry routes are through the skin, especially injured skin, the mucosal surfaces of other body openings, and the conjunctiva of the eyes. Pregnant mothers may also transmit infections to their fetuses. The degree of infection is related to the dose and virulence of the infecting agent, as well as to the resistance or immunity of the host against that organism. Resistance to infection is lowered by many diseases of the immune system, leukemia, and cancer, as well as by conditions such as severe burns or malnutrition. In healthy people, the body's own so-called natural flora of bacteria form a line of defense against mild infections, which may thus be taken care of without further treatment. Antibiotics such as sulfa drugs and other pharmaceuticals are used against more severe infections, and vaccines serve as preventive measures against a wide range of infectious diseases (see Immunization).

Among the many infectious diseases are the common cold, chicken pox, cholera, diphtheria, German measles, influenza, malaria, mumps, rabies, tuberculosis, and sexually transmitted infections. See also Disease.

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