pneumonia no͝omōnˈyə [key], acute infection of one or both lungs that can be caused by a bacterium, usually Streptococcus pneumoniae (also called pneumococcus; see streptococcus), or by a virus, fungus, or other organism. The causal organisms reach the lungs through the respiratory passages. Usually an upper respiratory infection precedes the disease. Alcoholism, extreme youth or age, debility, immunosuppressive disorders and therapy, and compromised consciousness are predisposing factors. When one or more entire lobes of the lung are involved, the infection is considered a lobar pneumonia. When the disease is confined to the air spaces adjacent to the bronchi, it is known as bronchopneumonia. Aspiration pneumonia is the pathological consequence of the abnormal entry of fluids, particulate matter, or secretions in the lower airways.

The symptoms of pneumonia are high fever, chills, pain in the chest, difficulty in breathing, cough, and sputum that is pinkish at first and becomes rust-colored as the infection progresses. The skin may turn bluish because the lungs are not sufficiently oxygenating the blood. Complete bed rest and good supportive care are important. Oxygen helps to relieve severe respiratory difficulty.

Immunization for pneumococcal pneumonia is recommended for children under two years old, adults 65 or older, and others at risk. Penicillin is most commonly used to treat pneumococcal pneumonia and other pneumonias caused by bacteria and, with the other antibiotic and sulfa drugs, is responsible for the marked decline since the mid-20th cent. in mortality figures. Nevertheless, pneumonia is still a serious disease, especially in elderly and debilitated persons (who usually acquire bronchopneumonia) or when complicated by bacterial invasion of the bloodstream, membranes of the heart, or the central nervous system.

Viral pneumonia, generally milder than the bacterial form, is the result of lower respiratory infection and has been the cause of more than 90% of deaths for individuals over 65. Pneumocystis carinii pneumonia, which is caused by an organism traditionally thought to be a parasitic protozoan but now suspected to be a fungus, generally only occurs in patients who have AIDS or leukemia or whose immune system is otherwise suppressed.

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