Atypical pneumonia

Atypical pneumonia, also known as walking pneumonia, is the type of pneumonia not caused by one of the pathogens most commonly associated with the disease. Its clinical presentation contrasts to that of 'typical' pneumonia. A variety of microorganisms can cause it. When it develops independently from another disease, it is called primary atypical pneumonia (PAP). Atypical pneumonia, also known as walking pneumonia, is the type of pneumonia not caused by one of the pathogens most commonly associated with the disease. Its clinical presentation contrasts to that of 'typical' pneumonia. A variety of microorganisms can cause it. When it develops 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, sweating and myalgia) and bronchopneumonia. 'Primary atypical pneumonia' is called primary because it develops independently of other diseases. 'Atypical pneumonia' is atypical in that it is caused by atypical organisms (other than Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis).These atypical organisms include special bacteria, viruses, fungi, and protozoa. In addition, this form of pneumonia is atypical in presentation with only moderate amounts of sputum, no consolidation, only small increases in white cell counts, and no alveolar exudate.At the time that atypical pneumonia was first described, organisms like Mycoplasma, Chlamydophila, and Legionella were not yet recognized as bacteria and instead considered viruses. Hence 'atypical pneumonia' was also called 'non-bacterial'. In literature the term atypical pneumonia (contrasted with bacterial pneumonia) is still in use, though incorrect. Meanwhile, many such organisms are identified as bacteria, albeit unusual types (Mycoplasma is a type of bacteria without a cell wall and Chlamydias are intracellular bacteria). As the conditions caused by these agents have different courses and respond to different treatments, the identification of the specific causative pathogen is important.