Atypical pneumonia is a term used to describe a disease caused by one or a combination of the following organisms:

Legionella pneumophila

Causes a severe from of pneumonia with a relatively high mortality rate.

Mycoplasma pneumoniae

Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.

Chlamydia pneumoniae

Mild form of pneumonia with relatively mild symptoms.

Symptoms include pyrexia, shortness of breath, laboured breathing, cough, arthralgia (joint pain), malaise, loss of appetite, confusion, rash, and diarrhoea

Treatment is with oral antibiotics and diagnosis is confirmed by blood cultures and sputum samples.

Prognosis is usually good and is influenced by age and immunosuppression.

The Severe Acute Respiratory Syndrome(SARS) was also considered a kind of atypical pneumonia, and is still called so in China.