Legionellosisis a respiratory infection caused by the Legionella bacteria. Severe cases of legionellosis take the form of Legionnaire’s Disease, a potentially deadly strain of pneumonia. Less serious legionellosis infection often takes the form of Pontiac Fever, which has milder symptoms similar to the flu.

Approximately 5,000 people are diagnosed with Legionnaire’s Disease every year in the United States. Infection occurs most often in the summer and early fall, but the disease can hit any time of year.

Here are some of the most commonly asked questions about the diagnosis, treatment, and prevention of Legionnaire’s disease.

  1. How do people get Legionnaire’s disease? The Legionella bacteria grows in freshwater environments; it grows best in warm water and can infect man-made water sources such as cooling towers, hot tubs, hot water tanks and water features, fountains, and large plumbing systems. Legionnaire’s disease spreads when people inhale small droplets of water contaminated with the bacteria. In less common cases, infection can occur through aspiration of contaminated water (i.e. when water “goes down the wrong pipe” and enters the trachea and lungs. Legionnaire’s disease typically does not spread from person to person, although this can occur in rare cases.
    Certain populations are more susceptible to Legionnaire’s disease; these include people over the age of 50, people with chronic lung diseases, people with weak immune systems, people on immunosuppressant drugs, and people who smoke or used to smoke.
  2. What are the symptoms of Legionnaire’s disease? Symptoms of Legionnaire’s disease mimic many other types of pneumonia and include cough, shortness of breath, fever, muscle aches, and headaches. The disease can also cause diarrhea, nausea, and disorientation or confusion. Symptoms typically begin within 2-10 days after exposure to the Legionella bacteria but can sometimes take up to two weeks to appear.
    Legionella bacteria can also cause Pontiac fever, which is characterized by fever and muscle aches. Pontiac fever is not a form of pneumonia.
  3. How is Legionnaire’s disease diagnosed? Pneumonia, including Legionnaire’s disease, can be confirmed by a chest x-ray. To determine whether a person’s pneumonia is caused by the Legionella bacteria, doctors will use two laboratory tests: a urine sample and/or a sputum (phlegm) sample.
  4. What is the typical treatment for Legionnaire’s disease? Legionnaire’s disease can be successfully treated with antibiotics. The prognosis for healthy people infected with Legionella bacteria is good, particularly if the infection is caught early. For patients with compromised or weakened immune systems, full recovery will often require hospitalization, however, and in some cases, the disease can lead to lung failure. About one out of ten people infected with Legionnaire’s disease will die of complications from the infection.
  5. How can I prevent Legionnaire’s disease? There are currently no vaccines to prevent Legionnaire’s disease or Pontiac fever. Proper maintenance of water systems, such water heaters and hot tubs, can reduce the risk of the Legionella bacterium growing and spreading.
  6. What kinds of diagnostic tests are used to test for Legionnaire’s disease? Diagnostic tests to detect Legionnaire’s disease include enzyme-linked immunosorbent assays (ELISA) and lateral flow immunochromatographic assays, like the IVD Legionella Urinary Antigen Microwell ELISA and the IVD Legionella Lateral Flow, which detect Legionella antigens.


Center for Disease Control (CDC). “Legionella (Legionnaire’s Disease and Pontiac Fever)”. Available at

Legionella.org. “What Is Legionnaire’s Disease?” Available at

Nordqvist, Christian. 2016. “What Is Legionnaire’s Disease?” Medical News Today. Available at