This article was last reviewed on
This article waslast modified on July 10, 2017.

Cases of Legionnaires disease, a serious, often fatal form of pneumonia, nearly quadrupled from 2000 to 2014, according to a recent report from the U.S. Centers for Disease Control and Prevention (CDC.)

Legionnaires disease is caused by species of Legionella bacteria, which are found naturally throughout the environment. These bacteria thrive in warm, stagnant water and can grow in the plumbing systems of large buildings such as hotels and hospitals as well as cruise ships. The bacteria can contaminate water sources such as hot tubs, drinking and bathing water, hot water tanks, air conditioning cooling towers, ice machines, humidifiers, and public fountains. People can contract Legionella by breathing in droplets of water that contain the bacteria.

Legionella was first discovered after an outbreak in 1976 among people who attended an American Legion convention in Philadelphia. Of about 2,000 people who attended the convention, 221 contracted the disease and 34 died. Now, about 5,000 people contract Legionnaires disease each year, and one in ten dies, according to the CDC.

Improved water disinfecting, better equipment, and better training of employees responsible for the disinfecting could have potentially prevented most of the Legionnaires disease outbreaks since 2000, according to the CDC report. "Better water system management is the best way to reduce illness and save lives…" said CDC Director Tom Friedan, M.D. M.P.H. Along with the report, CDC released a new toolkit to help building owners and managers prevent outbreaks of Legionnaires disease.

According to the report, the most common source of recent Legionnaires disease outbreaks associated with buildings was potable (drinkable) water used for common purposes, such as showering. This was followed by cooling towers, hot tubs, and decorative fountains.

While most healthy people don't develop Legionnaires disease after being exposed to Legionella, people who are more likely to develop the disease include those age 50 and older, current or former smokers, people with pre-existing lung disease, such as emphysema, and people with weakened immune systems. People at increased risk may choose to avoid potentially unsafe environments, such as hot tubs. They should seek medical care quickly if they develop symptoms of pneumonia and may be tested for Legionella.

Besides those at risk, other individuals may be tested for Legionella if, for example, they have severe pneumonia and are in intensive care, have been treated with antibiotics and are still ill, or have pneumonia during a Legionella outbreak. Legionella bacteria are not susceptible to the commonly prescribed antibiotics used to treat respiratory infections. Therefore, it is important to perform additional laboratory testing to detect Legionella if the patient is not improving on the initially prescribed antibiotics.

Common tests to check for Legionella include a urine test that detects a protein produced by Legionella bacteria and testing performed on sputum samples (deep respiratory secretions), which require the person being tested to cough up secretions (phlegm) from the lungs.


(June 7, 2016) Centers for Disease Control and Prevention. Press Release: Better water system maintenance needed to prevent Legionnaires' disease outbreaks. Available online at Accessed July 6, 2016.

(June 2016) Centers for Disease Control and Prevention. Vital Signs: Legionnaires' Disease. Available online at Accessed July 6, 2016.

Centers for Disease Control and Prevention. Legionella (Legionnaires' Disease and Pontiac Fever) History and Disease Patterns. Available online at Accessed July 6, 2016.

(June 7, 2016) Sun, Lena. Legionnaires' outbreaks: Cases nearly quadrupled in 15 years. The Washington Post. Available online at Accessed July 6, 2017.

(October 6, 2011) The Disease Daily. Legionnaires' Disease: Today and in 1976. Available online at Accessed July 2016.