Mycoplasmal, viral, and rickettsial pneumonias.

Of the variety of mycoplasma species, the smallest of free living organisms, only M. pneumoniae (Eaton agent) produces respiratory infections. These include pneumonia, tracheobronchitis, pharyngitis, and in 20% of cases, asymptomatic invasion . M. pneumoniae is one of a variety of organisms, including viruses, rickettsiae, and chlamydiae (psittacosis), that cause primary atypical pneumonia (PAP) . In young adults, M. pneumoniae is estimated to cause approximately 50% of PAP . Its transmission is by the respiratory route with the organisms appearing in sputum or throat washings several days before the onset of clinical infection and persisting for several weeks thereafter . Naturally acquired immunity ensues . Localized outbreaks in military or school populations, among families, or in general communities in cyclical periods of 4-5 yr are described . Although occurring at any season, more cases are seen in the fall and early winter. Again, the younger age population appears more at risk, with attack rates, within families, of 70% for children and 20% for adults . These infections are not uncommon in patients with chronic pulmonary disease."