The bacterium Legionella pneumophila is responsible for most cases of Legionnaires' disease. Outdoors, legionella bacteria survive in soil and water, but rarely cause infections. Indoors, though, legionella bacteria can multiply in all kinds of water systems — hot tubs, air conditioners and mist sprayers in grocery store produce departments.
Although it's possible to contract Legionnaires' disease from home plumbing systems, most outbreaks have occurred in large buildings, perhaps because complex systems allow the bacteria to grow and spread more easily.
How the infection spreads
Most people become infected when they inhale microscopic water droplets containing legionella bacteria. This might be the spray from a shower, faucet or whirlpool, or water dispersed through the ventilation system in a large building. Outbreaks have been linked to a range of sources, including:
- Hot tubs and whirlpools on cruise ships
- Cooling towers in air conditioning systems
- Decorative fountains
- Swimming pools
- Physical therapy equipment
- Water systems in hotels, hospitals and nursing homes
Although legionella bacteria primarily spread through aerosolized water droplets, the infection can be transmitted in other ways, including:
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- Aspiration. This occurs when liquids accidentally enter your lungs, usually because you cough or choke while drinking. If you aspirate water containing legionella bacteria, you may develop Legionnaires' disease.
- Soil. A few people have contracted Legionnaires' disease after working in the garden or using contaminated potting soil.
- Pedro-Botet ML, et al. Epidemiology and pathogenesis of Legionella infection. http://www.uptodate.com/home. Accessed June 26, 2013.
- Legionella (Legionnaires' disease and Pontiac fever). Centers for Disease Control and Prevention. http://www.cdc.gov/legionella/index.html. Accessed June 26, 2013.
- Longo DL, et al. Harrison's Online. 18th ed. New York, N.Y.: The McGraw-Hill Companies; 2012. http://www.accessmedicine.com/resourceTOC.aspx?resourceID=4. Accessed June 26, 2013.
- Pedro-Botet ML, et al. Clinical manifestations and diagnosis of legionella infection. http://www.uptodate.com/home. Accessed June 26, 2013.
- Mandell GL, et al. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, Pa.: Churchill Livingstone Elsevier; 2010. http://www.mdconsult.com/books/about.do?about=true&eid=4-u1.0-B978-0-443-06839-3..X0001-X--TOP&isbn=978-0-443-06839-3&uniqId=230100505-57. June 26, 2013.
- Pedro-Botet ML, et al. Treatment and prevention of legionella infection. http://www.uptodate.com/home. Accessed June 26, 2013.