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Legionella Infections

(Legionnaires' Disease)


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Feb 2020| Content last modified Feb 2020
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Legionella infections are caused by the gram-negative bacteria Legionella pneumophila and most often affect the lungs, causing pneumonia and flu-like symptoms.

  • The infection is often acquired by inhaling contaminated water droplets, as may be sprayed from shower heads or air-conditioning systems.

  • People have fever, chills, and muscle aches, and breathing may be difficult and painful.

  • Doctors identify the infection by analyzing samples of sputum, fluids from the lungs, or urine.

  • Antibiotics, such as fluoroquinolones, can effectively treat legionnaires' disease.

(See also Overview of Bacteria.)

Infection with Legionella bacteria was first identified in 1976 when there was a large outbreak of fatal pneumonia at a convention of the American Legion in Philadelphia, Pennsylvania. Thus, the infection was called legionnaires’ disease.

The following increase the risk of getting a Legionella infection:

Legionella bacteria are often present in soil and fresh water. Fresh water containing these bacteria may enter a building's plumbing system. Thus, a Legionella outbreak often begins in a building’s water supply. In such cases, people usually get the infection by inhaling contaminated water droplets that have been sprayed from shower heads, misters, whirlpool baths, or water cooling towers for air-conditioning.

Legionnaires' disease is not spread from person to person.

Legionella bacteria usually infect the lungs, causing legionnaires' disease.

Sometimes the bacteria affect only the upper respiratory tract and do not cause pneumonia. This infection is called Pontiac fever and is milder than legionnaires' disease.

Rarely, these bacteria can also infect other areas of the body, mainly in people with a weakened immune system or a serious illness. The heart is most commonly affected, but the brain and spinal cord, liver, spleen, lymph nodes, and intestines can also be affected.


Symptoms of legionnaires’ disease resemble the flu. People have fever, chills, a general feeling of illness, muscle aches, headache, and confusion. Other symptoms include nausea, loose stools or watery diarrhea, abdominal pain, cough, and joint pain. People may have difficulty breathing, and breathing may be painful. They may cough up blood.

With treatment, most otherwise healthy people recover. However, certain characteristics increase the risk of death:

  • Acquiring the infection in a hospital (up to half of infected people die)

  • Being older

  • Having a weakened immune system

Without treatment, about 5% of people with legionnaires' disease die, but this rate is higher (up to 40%) in people who are older, have a weakened immune system, or have acquired the infection in a hospital.

People with Pontiac fever have fever, headache, and muscle aches but no cough or other respiratory symptoms. Symptoms resolve on their own in about a week.


  • Culture and analysis of a sample of sputum or fluid from the lungs

  • Sometimes urine tests

To diagnose the infection, doctors take samples of sputum or fluid taken from the lungs and send them to a laboratory to grow (culture) and identify the bacteria. Polymerase chain reaction (PCR) testing may be done. It increases the amount of the bacteria's DNA and thus makes the bacteria easier to identify.

A sample of urine may be checked for specific substances produced by the bacteria (antigens). These tests cannot detect some types of Legionella bacteria, but they can detect the type that causes most infections.

A chest x-ray is taken to check for pneumonia.


  • An antibiotic

People with legionnaires' disease should be given an antibiotic. Usually, a fluoroquinolone, such as levofloxacin or moxifloxacin, is given intravenously or by mouth for 7 to 14 days and, if people have a greatly weakened immune system, sometimes up to 3 weeks. Other effective antibiotics include azithromycin, clarithromycin, and doxycycline.

People with Pontiac fever do not require antibiotics.

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