Rationale: There are many etiologies for atypical pneumonia, and they are often difficult to differentiate clinically. Mycoplasma is unique in that clinical pulmonary findings are often absent. Legionella often causes gastrointestinal symptoms and a severe headache. C. psittaci, Coxiella, and Legionella may have a specific history of exposure (e.g., birds, domestic animals, or environmental). Truly purulent sputum, as is seen in S. pneumoniae (a major cause of typical pneumonia), is not consistent with atypical pneumonia.