Learn About Legionnaires’ Disease

Key Facts:

  • Legionnaires’ disease is one type of bacterial pneumonia.
  • Legionella, the bacteria that causes the disease, thrives in warm water and is often spread through contaminated water systems.
  • Symptoms usually develop 2-10 days after being exposed.
  • In 2018, more than 10,000 cases of Legionnaires’ disease were reported in the United States. However, because it commonly goes undiagnosed, this number is probably underestimated.
  • A less severe form of the same disease is called Pontiac fever.

Who Is at Risk?

Not everyone exposed to Legionella will develop Legionnaires’ disease. Adults over the age of 50, current and former smokers as well as people with weakened immune systems are particularly susceptible to the disease after exposure to the bacteria. Having a chronic disease such as COPD, cancer, diabetes and kidney or liver failure can also put you at an increased risk of developing Legionnaires’ disease.

What Causes it?

The naturally-occurring bacteria Legionella can become a health concern when water is held in improperly maintained human-made structures and systems. In these instances, the bacteria breeds and multiplies, causing abnormally large amounts of Legionella to concentrate. The bacteria thrives in warm water and has been found in places such as shower heads, faucets, humidifiers, hot tubs and decorative fountains. The warm water can become a mist that, when breathed into the lungs, introduces bacteria from water into lung tissue.

Rarely, Legionnaires’ disease can also be contracted by aspiration while drinking contaminated water, if some of it accidentally enters the lungs by “going down the wrong pipe.”

A milder form of the same disease is called Pontiac fever.

Reviewed and approved by the American Lung Association Scientific and Medical Editorial Review Panel.

Page last updated: April 10, 2020

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