This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Legionnaires’ disease is a severe type of pneumonia caused by the bacteria Legionella. The species Legionella pneumophila causes most cases, but other species of Legionella can also cause the disease. It is named Legionnaires’ disease because it was first discovered after a pneumonia outbreak among people who attended an American Legion Convention in Philadelphia, Pennsylvania in 1976.
Most people exposed to Legionella do not become sick with Legionnaires’ disease. People who do become sick usually develop symptoms within 2 to 10 days after exposure, but it may take longer. The first symptoms may include headache, chills, muscle pains, and a fever that can be 104°F (40°C) or higher. Additional symptoms usually develop 1 to 2 days after the first symptoms and may include coughing, shortness of breath, chest pain, diarrhea, nausea and vomiting, and confusion. While Legionnaires’ disease mainly affects the lungs, it sometimes causes infections in other parts of the body, such as the heart or within body wounds.
A person can become infected from Legionella when they inhale mist or water droplets that contain the bacteria. Sources of exposure may include showers, faucets, whirlpools, grocery store misters, and water droplets passing through ventilation systems in large buildings (such as hotels, office buildings, and hospitals). People who are more susceptible to developing Legionnaires’ disease after an exposure include adults over age 50, current or former smokers, and people who have a weakened immune system or a chronic disease. Generally, neither the bacteria nor Legionnaires’ disease is spread directly from person to person. While large exposures can result in outbreaks, the disease usually occurs in single, isolated cases.
Legionnaires’ disease may be suspected by symptoms. Pneumonia can be confirmed by a chest X-ray. Legionnaires’ disease is diagnosed when one of the species of Legionella is found to be the cause of the pneumonia by testing a urine sample (urine culture) or a sample of saliva and mucus that is coughed up (sputum culture). Without treatment, the disease can be fatal. People with the disease who are otherwise healthy usually recover with antibiotics, although they may need to be cared for in a hospital. About 1 in 10 people with Legionnaires’ disease will not survive due to complications such as respiratory failure, kidney failure, or septic shock.
Of note, Legionella can also cause a milder illness called Pontiac fever, which causes flu-like symptoms, but does not cause pneumonia. Pontiac fever typically goes away without specific treatment.
For more information, visit GARD.