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Legionnaires Disease

Graphic: Montage of images, including Pennsylvania state seal, Time magazine cover, Newsweek magazine cover and the American Legion badge

The Legionella bacterium causes a type of pneumonia that got its name after the 1976 outbreak at an American Legion convention in Philadelphia.

The Basics

  • Identified in 1976
  • People can get Legionnaires’ disease by breathing in mist containing the bacteria.
  • In general, the bacteria are not spread from one person to another.
  • Symptoms usually begin 2–10 days after exposure.
  • There are no vaccines that can prevent Legionnaires’ disease.


  • Cough
  • Shortness of breath
  • High fever
  • Muscle aches
  • Headaches

Photo: Microscopic Image of the legionaella bacteria.

CDC is the nation's health protection agency, working 24/7 to protect America from foreign and domestic public health threats. Since its inception, the CDC has been committed to identifying and investigating new pathogens or causes of illness.

In 1976, CDC (in cooperation with other federal, state, and local authorities) launched one of the largest disease investigations in U.S. history following an outbreak of severe pneumonia (lung infection) among the participants of the American Legion Convention in Philadelphia, Pennsylvania. As a result, CDC identified the new bacterium (Legionella pneumophila) that was spread through the hotel’s air conditioning system. On July 27, three days after the convention ended, the first victim died. Within a week, more than 130 people, mostly men, had been hospitalized, and 25 had died. All had attended the convention and stayed at the same hotel.

About 5,000 cases of Legionnaires’ disease are now reported each year in the United States. People can get Legionnaires’ disease when they breathe in mist (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. Outbreaks are most commonly associated with buildings or structures that have complex water systems, like hotels, hospitals, long-term care facilities, and cruise ships. Within these structures, the bacterium can become a health concern when it grows and spreads in human-made water systems, like hot tubs, cooling towers, hot water tanks, large plumbing systems, and decorative fountains. They do not seem to grow in car or window air-conditioners. Most healthy people do not become infected with Legionella bacteria after exposure.

Who is at risk?

  • People 50 years or older
  • Current or former smokers
  • Those with a chronic lung disease (like COPD or emphysema)
  • Those with a weak immune system from diseases like cancer, diabetes, or kidney failure
  • People who take drugs that suppress (weaken) the immune system (like after a transplant operation or chemotherapy)


Legionnaires’ disease requires treatment with antibiotics (medicine that kills bacteria in the body), and most cases of Legionnaires’ disease can be treated successfully with antibiotics. Healthy people usually get better after being sick with Legionnaires’ disease, but hospitalization is often required. About 1 in 10 people who get Legionnaires’ disease will die from the infection.


The key to preventing Legionnaires’ disease is maintenance of the water systems in which Legionella grow, including drinking water systems, hot tubs, decorative fountains, and cooling towers.

of note...

Man relaxing in a hot tub

More illness is usually found in the summer and early fall, but it can happen any time of year. Additionally, many infections are not diagnosed, so the number affected may be higher.

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