Legionnaire Disease

Legionnaire Disease

Reviewed By:
Vikram Tarugu, M.D., AGA, ACG


Legionnaire disease isa type of pneumonia. It is one of a group of diseases (known collectively as legionellosis) that can occur after exposure to Legionella bacteria, most often Legionellapneumophila. Legionella bacteria are found naturally in the environment and are present in low levels in streams, rivers and ponds. However, these bacteria can grow to dangerous levels in the warmer water associated with hot water tanks, large air-conditioning systems, hot tubs and cooling towers. Breathing in the vapor of contaminated water can lead to Legionnaire disease or a milder form of infection called Pontiac fever.

The symptoms of Legionnaire disease often include moderate to severe pneumonia coupled with gastrointestinal problems such as diarrhea and vomiting. Because it is difficult to differentiate between Legionnaire disease and other forms of pneumonia, cases of Legionnaire disease are often underreported. Legionnaire disease can be treated with antibiotics.

Pontiac fever is a much milder infection of Legionella bacteria. It produces mild flu-like symptoms with no pneumonia. It is a self-limiting disease, which means that the bacteria stop multiplying after a certain amount of time.

Although there are few steps that individuals can take to avoid Legionella bacteria, the U.S. Centers for Disease Control and Prevention has suggested guidelines for building managers to reduce outbreaks. These include the regular maintenance, filtering and disinfecting of water storage and supply systems.

About Legionnaire disease

Legionnaire disease is a type of pneumonia caused by Legionella bacteria, most often Legionella pneumophila. Legionella bacteria can cause a range of illnesses that are collectively known as legionellosis. Legionnaire disease, which got its name after an outbreak of the disease occurred among hotel guests attending an American Legion meeting in Philadelphia in 1976, is the best known and most serious of these diseases. In that original outbreak, more than 200 people became ill and 34 people died. The bacteria that caused the illness was identified about six months later.

Another disease caused by the Legionella bacteria is Pontiac fever. Although this is more common than Legionnaire disease, it produces only mild symptoms and often clears of its own accord without treatment. Pontiac fever does not result in pneumonia.

The U.S. Centers for Disease Control and Prevention (CDC) estimates that between 8,000 and 18,000 people are hospitalized with Legionnaire disease in the United States every year. However, because Legionnaire disease is difficult to distinguish from other types of pneumonia, it may be difficult to diagnose and be underreported. The number of people infected each year with Legionnaire disease may be much higher than is currently estimated.

The fatality rate of Legionnaire disease may range from 5 to 30 percent, according to the CDC. It is even higher among hospitalized patients and people with compromised immune systems. According to the National Institutes of Health, the fatality rate can reach almost 50 percent among patients who develop the disease while hospitalized.  

Both forms of legionellosis (Legionnaire disease and Pontiac fever) are contracted by coming into contact with water particles that contain Legionella bacteria. Legionella bacteria occur naturally in the environment and can be found in ponds, streams and lakes. The bacteria also thrive in warm water such as that found in hot water tanks, large air-conditioning systems, hot tubs and cooling towers.

Legionellosis can occur when people breathe in contaminated water vapor from a hot tub or air conditioning system. It can also be contracted by drinking contaminated water. Unlike many types of pneumonia, Legionnaire disease cannot be spread from person to person.

Most healthy people exposed to the Legionella bacteria do not develop Legionnaire disease. The likelihood of a person contracting the disease after exposure to the bacteria depends on several factors, including:

  • Contamination level (amount of Legionella bacteria in water)

  • Susceptibility of person exposed to contaminated water

  • Intensity of exposure (e.g., how often or for how long exposed)

The disease is more prevalent in the summer and early fall, although it can occur any time of the year. Legionnaire disease is rare in children. Although outbreaks involving numerous people are sometimes reported in the media, the disease most often occurs sporadically, affecting one or two people at a time. When outbreaks occur, they tend to take place in large buildings that include living quarters, such as hotels or hospitals, where the bacteria may be spread from air conditioning systems or contaminated showerheads.

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