Atypical pneumonia

Atypical pneumonia refers to pneumonia caused by certain bacteria -- namely, Legionella pneumophila, Mycoplasma pneumoniae and Chlamydia pneumoniae . While atypical pneumonias have been commonly associated as a whole with milder forms of pneumonia, pneumonia due to Legionella , in particular, can be quite severe and lead to high mortality rates.

Alternative Names

Walking pneumonia

Causes, incidence, and risk factors

Atypical pneumonia due to Mycoplasma and Chlamydia usually cause milder forms of pneumonia and are characterized by a more protracted course of symptoms unlike other forms of pneumonia which can come on more quickly with more severe early symptoms. Mycoplasma pneumonia often affects younger people and may be associated with manifestations outside of the lungs (such as anemia and rashes), and neurological syndromes (such as meningitis, myelitis, and encephalitis). Severe forms of Mycoplasma pneumonia have been described in all age groups. Chlamydia pneumonia occurs year round and accounts for 5-15% of all pneumonias. It is usually mild with a low mortality rate. In contrast, atypical pneumonia due to Legionella accounts for 2-6% of pneumonias and has a higher mortality rate. Elderly individuals, smokers, and people with chronic illnesses and weakened immune systems are at higher risk for this type of pneumonia. Contact with contaminated aerosol systems (like infected air conditioning systems) has also been associated with pneumonia due to Legionella .

Signs and tests

People with suspected pneumonia should undergo a medical evaluation, including a thorough physical exam and a chest X-ray -- especially since the physical exam may not always distinguish pneumonia from acute bronchitis or other respiratory infections. Depending on the severity of illness, additional studies, such as a complete blood count, blood cultures, and sputum cultures, may be obtained. When certain forms of atypical pneumonia are suspected, certain tests (such as urine legionella antigen and throat swabs for Mycoplasma and Chlamydia , in addition to blood tests for antibodies to Mycoplasma and Chlamydia pneumonia) may be ordered.


The mainstay of treatment for atypical pneumonia is antibiotic therapy. In mild cases, treatment with oral antibiotics at home may be appropriate. Severe cases (especially common with pneumonia, due to Legionella ) may require intravenous antibiotics and oxygen supplementation. Antibiotics with activity against these organisms include -- erythromycin, azithromycin, clarithromycin, fluoroquinolones and their derivatives (e.g., levofloxacin), and tetracyclines (e.g., doxycycline).

Expectations (prognosis)

Most patients with pneumonia due to Mycoplasma or Chlamydia do well with appropriate antibiotic therapy, although there is a small chance of relapse if antibiotics are used for too short a period of time (less than two weeks). In the case of pneumonia due to Legionella , severe illness occurs particularly among the elderly and those individuals with chronic diseases and weakened immune systems. It is associated with a higher mortality rate.


  • Respiratory failure, mechanical ventilation -- especially in severe forms of pneumonia due to
  • Legionella
  • Rash and hemolytic anemia -- especially associated with pneumonia due to
  • Mycoplasma

    Calling your health care provider

    Seek medical evaluation if you develop fevers, cough, and/or shortness of breath. While there are numerous causes for these symptoms, you will need to be evaluated for pneumonia.


    There are no proven methods for preventing atypical pneumonia, and no vaccinations are available at this time for atypical pneumonias.

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