Pneumonia is an infection of the alveolar or gas-exchange portions of the lung. Some types of pneumonia produce an intense inflammatory response within the alveoli that leads to filling of the airspace with organisms, exudate, and white blood cells. Pneumonia can spread throughout the lung by the bronchial tree or through the pores of Kohn. Bacterial pneumonia, with an intense inflammatory response, tends to cause a productive cough, whereas other atypical organisms do not lead to such an intense inflammatory response and may only be associated with mild nonproductive cough.
Most of our knowledge about the etiology of pneumonia has been derived from studies of hospitalized patients where pneumococcus was the overwhelming cause. Prospective studies of both inpatients and outpatients with CAP often (up to 40 to 60 percent of the time) fail to identify a specific pathogen. 1 11 When an etiology is found, pneumococcus is still the most common single agent, followed by viruses and the atypical agents such as Mycoplasma, Chlamydia, and Legionella. In up to 5 percent, multiple agents are identified. Special populations, including nursing-home residents, chronic alcoholics, and HIV-infected patients, may have a somewhat different spectrum of disease.
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