|Classification and external resources|
|Specialty||Infectious disease, pulmonology|
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. 
Distinction between atypical and typical pneumonia is, however, medically insufficient. For the treatment of pneumonia, it is important to know the exact causal organism.
"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 parasites). As the conditions caused by these agents have different courses and respond to different treatments, the identification of the specific causative pathogen is important.
Signs and symptoms
Usually the atypical causes also involve atypical symptoms:
- No response to common antibiotics such as sulfonamide and beta-lactams like penicillin.
- No signs and symptoms of lobar consolidation, meaning that the infection is restricted to small areas, rather than involving a whole lobe. As the disease progresses, however, the look can tend to lobar pneumonia.
- Absence of leukocytosis.
- Extrapulmonary symptoms, related to the causing organism.
- Moderate amount of sputum, or no sputum at all (i.e. non-productive).
- Lack of alveolar exudate.
- Despite general symptoms and problems with the upper respiratory tract (such as high fever, headache, a dry irritating cough followed later by a productive cough with radiographs showing consolidation), there are in general few physical signs. The patient looks better than the symptoms suggest.
- Chlamydophila pneumoniae
- Mild form of pneumonia with relatively mild symptoms.
- Chlamydophila psittaci
- Causes psittacosis.
- Coxiella burnetii
- Causes Q fever.
- Francisella tularensis
- Causes tularemia.
- Legionella pneumophila
- Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires' disease.
- Mycoplasma pneumoniae
-  Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.
Atypical pneumonia can also have a fungal, protozoan or viral cause.
In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans.
When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial cause and is contrasted with viral pneumonia.
Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all. This is called occult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused by Streptococcus pneumoniae, as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.
Infiltration commonly begins in the perihilar region (where the bronchus begins) and spreads in a wedge- or fan-shaped fashion toward the periphery of the lung field. The process most often involves the lower lobe, but may affect any lobe or combination of lobes.
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