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Legionnaires' Disease

Last Updated: Mar 1, 2019


Legionnaires’ disease is a lung infection (pneumonia) that is caused by the Legionella pneumophila (commonly referred to as Legionella) bacterium. It is named after the first recorded outbreak at the 1976 American Legions Convention in Philadelphia, which led to 34 deaths. Legionnaires’ disease is contracted by inhalation and not transmitted from person to person. The bacteria live in warm water and soil, but can also thrive inside poorly sanitized water systems. Water sprays, air conditioning units, spas and other equipment that produce a fine mist can spread the bacteria and contaminate breathable air.

Legionella pneumophila bacteria

Legionnaires’ disease is fairly common in the United States and is linked to 8,000 - 18,000 hospitalizations each year. The majority of those contracting the disease do not become ill, while others require hospitalization and antibiotic treatment. Life-threatening pneumonia may develop in smokers, those with weakened immunity and other high-risk individuals. Similar to other types of pneumonia, Legionnaires’ disease has a 15% chance of death. The Legionella bacterium is also responsible for a milder flu-like disease called Pontiac fever, which does not affect the lungs and clears without treatment.

Causes and Risk Factors

Legionnaires’ disease is caused when Legionella bacteria enters the lungs through microscopic water droplets. Transmission usually happens though the nose (inhalation), and less often, the oral cavity (aspiration) - for example, when coughing reroutes some ingested fluid down the respiratory tract.

Legionella is found in soil and natural bodies of water, but in such low levels that there is little risk to humans. Water systems (in particular heated systems) that are not properly cleaned can harbor high levels of Legionella. When contaminated water is vaporized, sprayed or forced out through vents, it produces small droplets that can be inhaled. Water systems that can harbor and transmit Legionella include:

The body’s immune system can clear Legionella with little or no symptoms. However, roughly 5% of those who become exposed develop severe illness. The risk factors include:


Symptoms of Legionnaires’ disease begin anytime between two days to two weeks after exposure and are similar to those associated with other pneumonias.
Early symptoms include:

Late symptoms include:

Diagnosis and Treatment

Legionnaires’ disease is diagnosed by evaluating the lungs for pneumonia and confirming the Legionella infection.

Legionnaires’ disease is treated with antibiotics and often requires hospitalization. Early treatment helps prevent serious complications, including death.


Proper cleaning and disinfecting of pools, hot tubs, and other water systems that can harbor Legionella is the most effective way of preventing transmission. Avoiding smoking reduces the lung’s susceptibility to all pneumonias, including Legionnaires’ disease.


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Tina Shahian, PhD

Tina is a writer for Innerbody Research, where she has written a large body of informative guides about health conditions.


A communication specialist in life science and biotech subjects, Tina’s successful career is rooted in her ability to convey complex scientific topics to diverse audiences. Tina earned her PhD in Biochemistry from the University of California, San Francisco and her BS degree in Cell Biology from U.C. Davis. Tina Shahian’s Linkedin profile.


In her spare time, Tina enjoys drawing science-related cartoons.