Legionnaires' disease

Synonyms: Legionellosis, Pontiac fever

Medical Specialties: Family practice, Internal medicine, Pathology

Clinical Definition

Legionnaires' disease is an infection caused by the bacteria Legionella, found naturally in the environment and typically in warm water. The disease is transmitted via inhalation of droplets of water contaminated with the bacteria and typically can lead to pneumonia. It can be treated with antibiotics. 

In Our Own Words

Legionnaires’ disease is an infection caused by a bacterium that naturally lives in water and thrives in warm water. It’s no surprise then that the legionella bacteria can take root in warm, moist lungs to cause pneumonia.


The bacterium earned its name after an outbreak of the disease in 1976, when many people attending a convention of the American Legion were stricken with the disease. The bacteria are not spread person-to-person, but rather when people inhale a mist or small droplets of water (from contaminated water in hot tubs, fountains and water systems).


A milder form of Legionnaires' is known as Pontiac fever. Antibiotics are used to treat Legionnaires', but Pontiac fever typically gets better without specific treatment. 

Common Types
  • Legionnaires'
  • Pontiac fever
Side Effects
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  • Centers for Disease Control and Prevention. "Legionella (Legionnaires' and Pontiac Fever)." Updated February 2013. http://www.cdc.gov/legionella/about/index.html. Accessed August 2013.
  • American Lung Association. "Pneumonia." http://www.lung.org/lung-disease/pneumonia. Accessed August 2013.
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