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Causes and Transmission

Español: Causas y transmisión

Causes and Common Sources of Infection

Legionella is a type of bacterium found naturally in fresh water. When people are exposed to the bacterium, it can cause illness (Legionnaires’ disease and Pontiac fever). This bacterium grows best in warm water, like the kind found in

Man in hot tub

Legionella are NOT spread from one person to another person.

  • Hot tubs
  • Cooling towers (air-conditioning units for large buildings)
  • Hot water tanks
  • Large plumbing systems
  • Decorative fountains

Cooling towers use water to remove heat from a process or building. They are often part of the air conditioning systems of large buildings. In contrast, home and car air conditioning units do not use water to cool, so they do not aerosolize water (spread small droplets of water in the air) and are not a risk for Legionella growth.


Exposure and Transmission

People are exposed to Legionella when they breathe in a mist or vapor (small droplets of water in the air) containing the bacteria. One example might be from breathing in droplets sprayed from a hot tub that has not been properly cleaned and disinfected.

Less commonly, Legionella can be transmitted via aspiration of drinking water, which is when water “goes down the wrong pipe,” into the trachea (windpipe) and lungs instead of down the digestive tract. People at increased risk of aspiration include those with swallowing difficulties.

Legionella cannot spread from one person to another person. A person diagnosed with Legionnaires' disease or Pontiac fever is not a threat to family members, co-workers, or others. However, if you believe that your workplace was the source of the person's illness, contact your local health department.

Most people exposed to the bacteria do not become ill. If you have reason to believe you were exposed to the bacteria, talk to your doctor or local health department. Be sure to mention if you have spent any nights away from home in the last two weeks.

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