Atypical Pneumonia

Atypical pneumonia, also known as walking pneumonia, is a pneumonia not caused by one of the more traditional pathogens and with a clinical presentation that contrasts to the "typical" pneumonia. It can be caused by a variety of microorganisms. When developed independently from another disease it is called primary atypical pneumonia (PAP).

The term was introduced in the 1930s and was contrasted with the bacterial pneumonia caused by Streptococcus pneumoniae, at that time the best known and most commonly occurring form of pneumonia. The distinction was historically considered important as it differentiated those more likely to present with "typical" respiratory symptoms and lobar pneumonia from those more likely to present with "atypical" generalized symptoms (such as fever, headache, and myalgia) and bronchopneumonia.

Distinction between atypical and typical pneumonia, however, is medically insufficient. For the treatment of pneumonia it is important to know the exact causal organism. Moreover, S. pneumoniae has become a relatively less important cause.