Because of the limitations of diagnostic methods, the presence of a reliable etiological agent can be proved in only half of the community-acquired pneumonias affecting the general adult population. As shown in Tabje,2, the organisms identified most frequently are Strep. pneumoniae, M. pneumoniae, respiratory viruses, and Chlamydia pneumoniae. Coxiella burnetii has been reported to be prevalent in some rural areas. Other organisms, such as L. pneumophila, H. influenzae, Gram-negative aerobic bacilli, and Staph. aureus, are rare. In the case of severe community-acquired pneumonia, this etiological pattern shows some modifications (Table,, 2), as has been demonstrated in different studies using sophisticated or invasive methods. Strep. pneumoniae remains the most common pathogen, but the relative frequencies of L. pneumophila, H. influenzae, enteric Gram-negative bacilli, and Staph. aureus are substantially increased. In contrast, other etiologies such as M. pneumoniae, C. pneumoniae, and respiratory viruses become less relevant in this setting. L. pneumophila presents an important geographical and temporal variation. Occasionally, Mycobacterium tuberculosis may be responsible for severe community-acquired pneumonia.

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Table 2 Etiology of community-acquired pneumonia in the general population and in cases with severe presentation

The etiology of community-acquired pneumonia in patients from nursing homes has some peculiarities, particularly in those who are severely limited and handicapped. In such cases the etiological pattern may resemble that of nosocomial pneumonia, with enteric Gram-negative bacilli being the most common cause. Community-acquired pneumonia in patients who are immunosuppressed because of treatment for cancer or with long-term steroids, or in those with known HIV infection, may also be caused by a variety of opportunistic agents.

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