Atypical pneumonia refers to pneumonia caused by certain bacteria -- namely, Legionella pneumophila, Mycoplasma pneumoniae, and Chlamydophila pneumoniae.

While atypical pneumonias are commonly associated with milder forms of pneumonia, pneumonia due to Legionella, in particular, can be quite severe and lead to high mortality rates.

Atypical pneumonia due to Mycoplasma and Chlamydophila usually cause milder forms of pneumonia and are characterized by a more drawn out 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 symptoms 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.

Chlamydophila 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.

• Chills
• Fevers
• Cough
• Headache
• Muscle stiffness and aching
• Rapid breathing
• Shortness of breath
• Loss of appetite
• Malaise
• Confusion (especially with Legionella)
• Rash (especially with Mycoplasma)
• Diarrhea (especially with Legionella)

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, tests of your urine or a throat swab may be ordered as well.