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Numerous prospective studies examining the etiology for community-acquired pneumonia (CAP) have found that in a significant proportion of cases there is no identifiable cause. This is due, in part, to incomplete or inadequate diagnostic evaluation of all potential pathogens. Although CAP is traditionally divided into typical and atypical pneumonia based on clinical syndromes due to “typical” pathogens (eg, Streptococcus pneumoniae) versus “atypical” organisms (eg, Mycoplasma pneumoniae,Chlamydia pneumoniae, and Legionella pneumophila), it has been repeatedly shown that this dichotomous classification lacks enough predictive value and discriminatory ability to be reliable. As a result, the initial treatment of CAP remains largely empiric. Nevertheless, the term “atypical” is still pervasive in the medical lexicon and useful for discussion. The three major atypical organisms that account for pneumonia and upper respiratory infections are resistant to β-lactam antibiotics: C pneumoniae and L pneumophila because they are intracellular and M pneumoniae because it lacks a cell wall. This chapter discusses these and also discusses some of the important viral causes of CAP, specifically adenovirus, hantavirus, influenza, and varicella pneumonias and Severe Acute Respiratory Syndrome. The key clinical and laboratory features of each of these entities are summarized in Table 37–1. Respiratory viruses such as measles, respiratory syncytial virus, parainfluenza viruses, coxsackievirus, enteroviruses, cytomegalovirus, human metapneumovirus, and herpes simplex viruses are not discussed.

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Table 37–1. Key Features of Viral and Atypical Bacterial Pneumonias.

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