Legionnaires' Disease

Legionnaires’ (LEE-juh-NAIRS) disease, also known as legionellosis (LEEjuh-nel-O-sis), is a bacterial infection that can lead to a serious form of pneumonia (nu-MO-nyah), or inflammation of the lungs.

What Is Legionnaires' Disease?

In 1976, more than 200 people attending an American Legion convention at the Bellevue-Stratford Hotel in Philadelphia, Pennsylvania, suddenly came down with a mysterious illness that caused high fever, chills, and a cough. Thirty-four people died from severe pneumonia. The illness, later named Legionnaires’ disease in memory of the convention attendees, was caused by a previously undiscovered bacterium that scientists called Legionella pneumophila (LEE-juh-NEL-uh new-MOH-fee-luh). The bacteria were found to be living in the hotel's air-conditioning system. Since the initial outbreak, several other species of Legionella bacteria have been discovered, some of which can cause a very similar but less serious disease. Although people hear about outbreaks of Legionnaires' disease in public places that affect many people at once, such as the 2014 outbreak in Portugal or the 2015 outbreak in the cooling towers of the Lincoln Hospital in the South Bronx, New York, outbreaks actually occur more frequently on a much smaller scale. The disease can even break out in homes.

How Common Is Legionnaires' Disease?

* lung conditions such as emphysema * may be at especially high risk. People with weakened immune systems * , such as those who have AIDS * or cancer * , or those who have undergone organ transplantation, are also at greater risk of contracting the disease.

Scanning electron micrograph (SEM) depicted a grouping of Gram-negative Legionella pneumophila bacteria.

Scanning electron micrograph (SEM) depicted a grouping of Gram-negative Legionella pneumophila bacteria.
CDC/Janice Haney Carr.

How Does Legionnaires' Disease Spread?

Legionnaires’ disease is not spread from person to person. Legionella bacteria live and grow in warm, stagnant (still) water, such as that found in airconditioning systems, hot-water tanks, or whirlpool spas. People might become infected by breathing in the mist from contaminated water sources (for example, the vents of air conditioners at a hotel or the showers at a gym).

What Are the Signs and Symptoms of Legionnaires' Disease?


When Legionnaires’ disease broke out at the American Legion convention in Philadelphia in the summer of 1976, theories about its cause ranged from swine flu to chemical poisoning to communist conspiracies against American veterans. As fears mounted, the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, in cooperation with other federal, state, and local authorities, launched one of the largest joint disease investigations in history. Investigators looked at the possible sources of the outbreak and the ways the infection could have spread. Survivors were examined; bodies were autopsied; and specimens of air, water, soil, and various hotel materials were tested. Comparing tissue samples from people who had become infected with samples held in storage at the CDC, investigators were even able to link the Philadelphia cases to unidentified cases of illness dating back as far as the 1940s. Finally, in 1977, doctors isolated the culprit bacterium from the hotel's cooling tower, and federal authorities put in place new hygiene safeguards to limit future problems. The CDC continued thereafter to work in disease prevention and control, addressing such wide-ranging topics as air pollution, water contamination, and unsafe working conditions as well as implementing educational programs and various other strategies to protect the health of Americans.

How Do Doctors Diagnose and Treat the Disease?

Legionnaires’ disease can be difficult to distinguish from other types of pneumonia. There are very specific tests for it, but before ordering these tests, doctors need to learn as much as possible about their patients. First, doctors ask patients about their general health and recent activities. This history helps determine whether the patient might have been exposed to Legionella bacteria. Doctors then perform a physical examination of patients and take chest x-rays to look for signs of pneumonia in the lungs. If a doctor suspects Legionnaires' disease at this time, more specific laboratory tests might be ordered, including blood tests, to determine whether the patient's body is producing antibodies * to Legionella bacteria. Cultures of fluids from the patient's lungs may be done. To perform a culture, a person's sputum is placed on special material called a culture medium. If Legionella bacteria are present in the sputum, the medium will help them grow so that the bacterium can be identified. A urine test can also help confirm the presence of infection. The disease is treated with antibiotics and usually requires a stay in the hospital. In the hospital, patients receive supportive care, such as oxygen if they are having trouble breathing and extra fluids to replace what has been lost during periods of high fever.

What Are the Complications of Legionnaires' Disease?

Some people with Legionella infection experience only mild symptoms of illness, whereas others may be hospitalized for several weeks. Afterward, they may continue to be very tired for several more months. Most people who become ill with Legionnaires' disease recover, but in people with chronic lung problems, Legionella can make the condition worse, leading to severe illness. Up to 30 percent of cases of Legionnaires' disease are fatal.

Can Legionnaires' Disease Be Prevented?

There is little that people can do to avoid becoming infected with Legionella. In public places, better maintenance of plumbing and air-conditioning systems, including regular inspection and cleaning, can help limit the growth of the bacteria. If there is an outbreak, government health teams step in to decontaminate the suspected source of the bacteria.

See also Bacterial Infections • Pneumonia


Books and Articles

Freije, Matthew R. Protect Yourself from Legionnaire's Disease: The Waterborne Illness that Continues to Kill and Harm. San Diego, CA: HC Information Resources, 2011.

Mueller, Benjamin. “Legionnaires’ Bacteria Regrew in Bronx Cooling Towers That Were Disinfected.” The New York Times. October 1, 2015.


MedlinePlus. “Legionnaires’ Disease.” U.S. National Library of Medicine. http://www.nlm.nih.gov/medlineplus/legionnairesdisease.html (accessed November 17, 2015).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: http://www.cdc.gov (accessed November 17, 2015).

Legionella. Website: http://legionella.org (accessed November 17, 2015).

* chronic (KRAH-nik) means lasting a long time or recurring frequently.

* emphysema (em-fuh-ZEE-mah) is a lung disease in which the tiny air sacs in the lungs become permanently damaged and are unable to maintain the normal exchange of oxygen and other respiratory gases with the blood, often causing breathing difficulty.

* immune system (im-YOON SIStem) is the system of the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.

* AIDS or acquired immunodeficiency (IH-myoo-no-dih-FIH-shen-see) syndrome, is an infection that severely weakens the immune system. It is caused by the human immunodeficiency virus (HIV).

* cancer a condition characterized by the uncontrolled overgrowth of abnormal cells, which may be fatal.

* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)