The term “atypical pneumonia” was first termed over 60 years ago to describe cases of pneumonia caused by an unknown agent(s) and which appeared clinically different from pneumococcal pneumonia. It was initially characterized by constitutional symptoms, often with upper and lower respiratory tract symptoms and signs, a protracted course with gradual resolution, the lack of typical findings of consolidation on chest radiograph, failure to isolate a pathogen on routine bacteriologic methods, and a lack of response to penicillin therapy. In the 1940s an agent that was believed to be the principal cause was identified as Mycoplasma pneumoniae. Subsequently other pathogens have been linked with atypical pneumonia because of similar clinical presentation, including a variety of respiratory viruses, Chlamydia pneumoniae, Chlamydia psittaci, and Coxiella burnettii. Less common etiologic agents associated with atypical pneumonia include Francisella tularensis, Yersinia pestis (plague), and the Sin Nombre virus (hantavirus pulmonary syndrome), although these agents are often associated with a more acute clinical syndrome. In addition, although presently exceedingly rare, inhalation anthrax is included in part because of the concern for this pathogen as an agent of bioterrorism. Finally, pneumonia caused by Legionella species, albeit often more characteristic of pyogenic pneumonia, is also included since it is not isolated using routine microbiologic methods.
Although the original classification of atypical and typical pneumonia arose from the perception that the clinical presentation of patients was different, recent studies have shown there is excessive overlap of clinical manifestations of specific causes which does not permit empiric therapeutic decisions to be made solely on this basis. Thus, the designation of atypical pneumonia is controversial in relation to scientific and clinical merit; and many authorities have suggested that the term “atypical” be discontinued. However, the term remains popular among clinicians and investigators and remains prevalent in recent literature regardless of its clinical value. Moreover, options for appropriate antimicrobial therapy for the most common causes are similar, which is considered justification by some to lump these together.