Legionnaires’ disease

Legionnaires’ disease is a severe type of pneumonia caused by the bacteria Legionella.^[1] 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.^[2]

Most people exposed to Legionella do not become sick with Legionnaires' disease.^[3] 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.^[1] 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.^[1][4] 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.^[1]

A person can become infected from Legionella when they inhale mist or water droplets that contain the bacteria.^[1][3] 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).^[1] 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.^[1][3] Generally, neither the bacteria nor Legionnaires' disease is spread directly from person to person.^[5][6] While large exposures can result in outbreaks, the disease usually occurs in single, isolated cases.^[7]

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).^[8] Without treatment, the disease can be fatal.^[1] 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.^[1][8]

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.^[8]

Related diseases are conditions that have similar signs and symptoms. A health care provider may consider these conditions in the table below when making a diagnosis. Please note that the table may not include all the possible conditions related to this disease.

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