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:

  • 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.
Jan. 02, 2014

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