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Santé Montréal

Legionnaires' Disease

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Legionnaires’ disease is a disease caused by a bacterium called Legionella. Legionnaires’ disease is the most frequent form of legionellosis and the most serious, because it causes an acute lung infection (pneumonia)

Sources of infection

Legionella bacteria are very widespread in the environment. They occur naturally in fresh water (lakes, rivers, streams) and wet soils. However, they mostly occur in a variety of artificial water sources, especially hot water.

The main sources of infection are the following:

  • Drinking water distribution systems (especially hot water): hot water heaters, showerheads and sink faucets
  • Water cooling towers
  • Spas, hot tubs, whirlpools and Jacuzzis
  • Home humidifiers
  • Respiratory treatment equipment (e.g. CPAP) 


Legionnaires’ disease is not a common disease and, for people in good health, the risks of getting it are quite low. People can get the disease when they breathe in fine water droplets suspended in air contaminated with the Legionella bacteria. Legionnaires' disease does not spread from person to person.

People can be exposed to the bacteria in the community (e.g. at home), at work (e.g. jobs related to maintenance of air conditioning systems), on trips, or in healthcare settings (e.g. inhalation therapy equipment, humidifiers).

People at risk

Some people are more likely to develop the infection. Most often, those affected are:

  • People aged over 50
  • Smokers
  • Heavy drinkers
  • People with chronic diseases (e.g. kidney, lung or heart disease, diabetes)
  • People with weakened immune systems (e.g. cancer,  cancer treatment, transplant
  • People with recent surgery 

See the recommendations for Preventing Legionnaires’ disease at home (PDF)

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Legionnaires’ disease

Symptoms of Legionnaires’ disease (pneumonia caused by the legionella bacteria) usually begin 2 to 14 days following exposure to the bacteria, and up 20 days in people with weakened immune systems.

The most common symptoms associated with legionellosis are:

  • High fever
  • Chills
  • Cough
  • Fatigue
  • Muscle pain
  • Loss of appetite

The diagnosis is made on the basis of lab tests. Legionellosis can be treated with antibiotics.


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When to Consult

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Legionnaires' disease is a serious illness that typically requires hospitalization. In general, the mortality rate is one to two out of every ten people infected. However, the proportion can increase with age or presence of chronic illnesses, and in hospitalized patients.


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Protection and prevention

At home

  • Maintain the temperature of your hot water heater at a minimum of 60 °C.
  • Reduce the risks of infection further through proper maintenance of devices that cause water droplets to be sprayed into the air (e.g. showerheads, whirlpool baths, spas, humidifiers), in accordance with the manufacturer's instructions or the advice of a qualified professional.

    See the recommendations for Preventing Legionnaires’ disease at home (PDF)

In large buildings

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Risk factors

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Help and Resources

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People at Risk

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Dernière mise à jour le : 2020.07.03