Legionellosis (Legionnaires' Disease) Fact Sheet

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Legionnaires’ disease is a kind of bacterial pneumonia

Although there have been documented cases of this form of pneumonia that occurred as far back as 1947, the germ (bacterium) that causes it was not identified until 1976, when a number of cases occurred in Philadelphia among people attending an American Legion convention. The disease was then named after this outbreak. The bacterium was later named Legionella pneumophila.

The illness caused by the bacteria may vary in severity

Legionnaires’ disease causes pneumonia that can sometimes be severe and lead to death. More serious illness tends to occur in men over 50, smokers, people with diabetes mellitus, chronic lung disease, or kidney disease. People with an underlying cancer or immune problem may also be at increased risk of Legionnaires’ disease.

The Legionella bacterium is common in the environment

It can be easily found in aqueous (water) environments, such as air conditioning cooling towers, hot and cold water taps, showers, humidifiers, whirlpool spas, creeks, and ponds.

People get Legionnaires’ disease from inhaling contaminated water particles

The Legionella bacterium is spread by the release of small droplets of contaminated water into the air from air conditioning cooling towers, showers, misters, humidifiers, etc. To cause illness, infected water droplets must be inhaled (breathed in) by a susceptible person. The disease is not spread from person to person.

Symptoms usually occur 2 to 10 days after coming in contact with Legionella and may include:

  • High fever
  • Chills
  • Dry cough
  • Shortness of breath
  • Muscle aches
  • Diarrhea
  • Fatigue
  • Headache
  • Abdominal pain

Legionnaires’ disease is treated with antibiotics

The health department may investigate certain cases of Legionnaires’ disease

If more than one case of Legionnaires’ disease occurs from a common exposure, the health department may look for a possible environmental source of contaminated water.