How is Legionnaires’ Disease Transmitted?

Legionnaires’ disease, a severe form of pneumonia, is caused by bacteria called Legionella. The bacteria are not spread from one person to another person.  Instead, they are spread primarily by water, but can colonize in any warm, damp environment.

  • Inhalation.Many cases of Legionnaires’ disease are caused by people breathing in mist containing Legionella bacteria. These cases are often associated with showers, faucets, whirlpools, swimming pools, fountains and cooling towers in air conditioning systems.
  • Aspiration. These cases occur when contaminated water or other liquid enter the lungs.  Hospital-acquired Legionnaires’ disease is sometimes due to aspiration of Legionella that has contaminated equipment.
  • Soil. A few people have contracted Legionnaire’s disease after working in the garden or using contaminated potting soil. It’s also possible that the disease may spread when earth containing the bacteria is stirred up at large construction sites.

Legionnaires’ disease outbreaks happen when two or more people become ill after having spent time in the same place at about the same time.  Outbreaks are often associated with hospitals, nursing homes, hotels, and cruise ships.


Signs and symptoms of Legionnaires’ disease usually begin 2 to 14 days after exposure to the bacteria (Legionella) and can include the following:

  • High fever
  • Chills
  • Cough (may bring up mucus and blood)
  • Shortness of breath
  • Muscle aches
  • Headaches
  • Chest pain
  • Fatigue
  • Loss of appetite
  • Nausea
  • Vomiting
  • Diarrhea
  • Confusion

Prompt treatment is critical.  Legionnaires’ disease can be fatal, and some survivors never fully recover.

Legionnaires’ Disease Tests

When a patient has pneumonia, tests can determine if the patient has Legionella pnuemonia:

  • Urinary Antigen Test.  This test detects Legionella bacteria from a urine sample.
  • Cultured Bacteria.  If Legionella bacteria is cultured (isolated and grown on a special media) from a lung biopsy specimen, respiratory secretions, or various other sites, the diagnosis of Legionnaires’ disease is considered confirmed.
  • Blood Test.  Paired sera (blood specimens) that show a specific increase in antibody levels when drawn shortly after illness and several weeks following recovery, can also be used to confirm the diagnosis.


Legionnaires’ disease complications include, but are not limited to, the following:

  • Respiratory failure. This is one of the top causes of death in patients with pneumonia, including Legionnaires’ disease. If there are mechanical changes in the lungs (ventilatory failure) or oxygen loss in the arteries (hypoxemic respiratory failure), respiratory failure may result.
  • Septic shock. Toxins released by the Legionella bacteria may lead to a sudden drop in blood pressure, which reduces blood flow to vital organs, including the heart and kidneys.  This can result in multiple organ failure and death.
  • Acute kidney failure. This is the sudden loss of your kidneys’ ability to eliminate excess fluid and waste from the blood. Dangerous levels of fluid and waste accumulate in the body, which can lead to death.
  • Endocarditis. This is an infection of the endocardium, an inner lining of the heart. This can happen with the Legionella (the bacteria that cause Legionnaires’ disease) spread through the bloodstream.  Endocarditis can damage heart valves.
  • Pericarditis. This is a swelling and irritation of the pericardium, the membrane around your heart.  Pericarditis can cause long-term illness.

Legionnaires’ Disease Information