Legionnaires' Disease

Reviewed on 10/21/2019

Legionnaires' disease and Pontiac fever (legionellosis) facts

X-ray image of lungs of patient with Legionnaires' disease
X-ray image of lungs of patient with Legionnaires'; disease
  • Legionellosis is an infection that is caused by a bacterium.
  • The bacterium thrives in droplets of water and moist conditions usually associated with water systems.
  • The bacterium can infest an entire building.
  • The illness takes two distinct forms: Legionnaires' disease and Pontiac fever.
  • Legionnaires' disease is the more severe form and can be fatal.
  • Pontiac fever is the far milder form of the illness.
  • Symptoms of Legionnaires' disease include fever, chills, and a cough.
  • At its worst, Legionnaires' disease can cause severe pneumonia and respiratory failure.
  • Although antibiotics are an effective medical treatment, the most useful approach is prevention with the maintenance of water systems.


Legionellosis is an infection that is caused by a Legionella species called Legionella pneumophila. The disease has two distinct forms:

  • Legionnaires' disease is the more severe form of the infection, which may involve pneumonia. The onset of this form of the disease is usually two to 10 days after infection but can occur up to 16 days later. Legionnaires' disease acquired its name in 1976 after an outbreak of pneumonia occurred among people attending the American Legion convention at the Bellevue-Stratford Hotel in Philadelphia. Later, the bacterium causing the illness was named Legionella pneumophila.
  • Pontiac fever is a milder illness that develops from hours to two days after initial infection and resolves spontaneously. Sometimes, it may not even cause symptoms.

Where is the Legionella bacterium found?


The bacterium thrives in the mist aerosolized from air-conditioning ducts. Thus, it can infest an entire building or airplane. Factors that are now known to enhance the growth of Legionella bacteria in human-made water environments include the following:

  • Water temperatures of 77 F-107.6 F
  • Stagnation of the water
  • Scale and sediment in the water
  • Certain free-living amoebae organisms in water capable of supporting intracellular growth of Legionellae

What are risk factors for Legionnaires' disease?


People at an increased risk for Legionnaires' disease also include people whose immune systems are suppressed by diseases and conditions such as cancer, kidney failure requiring dialysis, diabetes, or AIDS. Those who take medications that suppress the immune system are also at risk including those chronically on steroids.

Pontiac fever most commonly occurs in healthy people.


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How is Legionnaires' disease spread? How does a person get Legionnaires' disease?

Outbreaks of legionellosis have occurred after people have inhaled aerosols that come from a water source (for example, air-conditioning cooling towers, hot tubs, or showers) contaminated with Legionella bacteria. People may be exposed to these aerosols in homes, workplaces, hospitals, or public places. Person-to-person infection does not occur with legionellosis, and there is no evidence of people becoming infected from vehicle air conditioners or household window air-conditioning units.

Cases of Legionnaires' disease have increased throughout the United States, but this most likely represents increased detection by better diagnostic tests. Some would suggest that this increase may be due to global warming, though this seems unlikely.

Hospital-acquired (nosocomial) infections have occurred and are usually due to contamination of the water supply. Two well-reported cases occurred in the 1980s in Los Angeles, one at the Wadsworth VA hospital and the other at the City of Hope. Guidelines have now been established for routine environmental testing for Legionnaires' in several states in the United States and by the Veterans Affairs health care system.

With regard to travel-associated infection, this usually occurs in hotels from contaminated water, most notably at the Legionnaires' convention in Philadelphia. This disease has also been associated with travel on cruise ships.

Some data suggest an increase incidence of Legionnaires' in the elderly in nursing homes linked to eating puréed food, probably also due to increased aspiration in this population.

How common is Legionnaires' disease?


Outbreaks of Legionnaires' disease have received the most media attention. A disease outbreak was associated with a party at the Playboy mansion in Los Angeles where at least four individuals contracted Legionnaires' disease. However, the disease most often occurs as single, isolated cases not associated with any identified outbreak. Outbreaks are usually recognized in the summer and early fall, but cases may occur all year long. More recently, outbreaks have been detected in gym participants in Florida and in a police station in New York.

Since the bacterium of Legionnaires' disease was identified in 1976, numerous hospital-acquired outbreaks of the disease have also been reported. These outbreaks have enabled health officials to study epidemics of legionellosis. A recent CDC evaluation reported that one in five Legionnaires' disease cases are linked to health care facilities.

What are the usual symptoms of Legionnaires' disease?


People with Pontiac fever experience a self-limiting influenza-like illness with fever, chills, headache, and muscle aches but, by definition, do not have pneumonia. Sickened individuals generally recover in two to five days without treatment.


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How is the diagnosis of Legionnaires' disease made?




What are medical treatments for Legionnaires' disease?

There are three major classes of antibiotics that are effective in treating a Legionella infection. These include the fluoroquinolones such as levofloxacin (Levaquin), and moxifloxacin (Avelox), the macrolides such as erythromycin, azithromyocin (Zithromax), and clarithromycin (Biaxin), and the tetracyclines including doxycycline (Vibramycin). A new class of antibiotics (glycylcyclines) are also effective. The choice of antibiotic is often dependent on the patient's clinical state, tolerance to the medication, and a health care professional's degree of certainty as to the diagnosis. Zithromax and Levaquin are particularly effective because of decreased gastrointestinal irritation, higher potency, better penetration into tissue, and once-daily dosing.

In severe cases of Legionnaires' disease that seem more resistant to a single antibiotic, a second drug called rifampin (Rifadin) may be added.

Pontiac fever requires no specific treatment.

Is it possible to prevent Legionnaires' disease?




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Castillo, Natalia E., Arun Rajasekaran, and Sayed Ali. "Legionnaires' Disease: A Review." Infectious Diseases in Clinical Practice 24.5 September 2016: 248-253. <http://journals.lww.com/infectdis/Abstract/2016/09000/Legionnaires__Disease__A_Review.2.aspx>.

United States. Centers for Disease Control and Prevention (CDC). "Legionella (Legionnaires' Disease and Pontiac Fever." April 30, 2018. <https://www.cdc.gov/legionella/about/history.html>.

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