Legionnaires Disease: Know the Facts


Legionnaires disease, also known as Legionellosis, is a potentially deadly form of pneumonia that is caused by the Legionella bacterium.

Legionnaires is normally spread by breathing in aerosolized water droplets (mist). This mist is commonly created by the hot tubs, air conditioning systems in large buildings or hotels, and grocery store mist machines. The disease generally cannot be transmitted between people.

Studies have estimated that Legionella causes 2-9% of pneumonia cases each year. There are an estimated 8,000 - 18,000 (hospitalizations) cases of Legionnaires' disease in the U.S. each year. Meaning, there are approximately 30 infections of Legionnaires per 100,000 residents per year. The frequency of infections peaks during the summer months and in warmer climates. However, it is estimated that 90% of instances of Legionnaires disease is misdiagnosed in patients because it shares similar symptoms with common pneumonia.

According to a 2011 article, Massachusetts had 211 reported cases of the disease in 2011 and 118 in 2010. While Connecticut had 72 and 47, respectively.

There is no vaccine for Legionnaires disease. Its prevention depends on the maintenance of water systems. 10% of those infected with the disease die. While hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the principal source of infection in such cases is the drinking-water distribution system.

Certain risk factors increase a person's risk of contracting Legionnaires disease. These include: being a current or former smoker, having chronic lung disease, having a weakened immune system, or having a history of pneumonia. Respiratory failure, septic shock, or acute kidney failure may result if Legionnaires disease goes untreated.

Radiologic imagining alone of the lung cannot distinguish Legionnaires disease from a normal form on pneumonia. Instead, further bacterial testing must be completed to verify the diagnosis through the analysis of coughed up mucus (sputum) or urine samples.


Legionella pneumophila is an aerobic, Gram-negative bacterium that causes Legionnaires disease.

The bacterium is naturally found in freshwater bodies such as lakes and streams. It can contaminate hot water tanks, hot tubs, large plumbing systems, fountains, and cooling towers of air conditioners.


The first and most famous outbreak of Legionnaires disease occurred in 1976 in Philadelphia, PA, at the Bellevue-Stratford Hotel. 221 American Legion members contracted the disease and 34 died. After an unprecedented investigation, the Centers for Disease Control and Prevention discovered that the Legionella bacterium was breeding inside the hotel's air conditioning systems. Prior to this outbreak and discovery, scientists believed the Legionella bacterium only affected animals. As such, this discovery lead researchers to look back at prior undiagnosed outbreaks that exhibited similar symptoms, resulting in the researchers concluding that many cases were caused by Legionnaires.

The world's largest outbreak of Legionnaires happened in 2001 at a hospital in Murcia, Spain. There were 636-696 confirmed cases, with 6 deaths. Recently, between June 2015 and January 2016, 87 cases of Legionnaires disease have occurred in Flint, Michigan, during the ongoing water crisis. 10 of those who contracted the disease died.


Levoflaxacin (Levaquin) and azithromycin (Zithromax) are the two standard treatment options for Legionnaires disease in the United States.


  • AIHA 2015 Guidelines
  • ASHRAE 2015 Standards


Because Legionnaires is a threat to large commercial water systems, a systematic water safety plan should be developed by entities to prevent the spread of the disease. For example, the plan should include:

  • Keeping water temperatures below or above 68-122 degrees F.
  • Preventing stagnation of water systems through better design methods.
  • Disinfecting water systems through high heat or chemical treatments.

Legionella dies within 2 minutes in temperatures above 151 degrees F and becomes dormant at temperatures below 68 degrees F.

Companies have also developed anti-microbial cooling towers to prevent the growth of Legionella and other bacteria. For example, the Delta Anti-Microbial Tower.


Water and air testing are the two primary methods for the detection of Legionella. Water testing is the primary means to detect Legionella in systems. Samples are collected through a water testing kit and sent to a certified microbiological laboratory for analysis. However, microbial counts or concentrations do not correlate with infectivity. Air samples are collected on special culture plates, but this method rarely detects the bacteria. There is great variability in the amount of airborne contaminated water droplets. Therefore, air sampling for legionella is rarely done.

Legionellosis is a reportable disease. Outbreaks of Legionnaires should be reported to the CDC's "Waterborne Disease Outbreak Surveillance System."

The European research collaborative, POSEIDON, highlighted in the December 2016 ASHRAE Journal, invented a scanner that detects Legionella bacteria in one hour, opposed to the 10 days it currently takes to detect the bacteria. The scanner uses a laser sensor to determine the existence of the bacteria. The organization believes it will have the product to market in three years.


Pontiac Fever is a similar, non-fatal respiratory disease caused by various specious of the Legionella bacterium. It causes a mild upper respiratory infection that resembles acute influenza. As such, the disease very often goes undiagnosed.

As with Legionnaires disease, the bacterium that causes Pontiac Fever resides in contaminated water systems.

The disease is treated with antibiotics and resolves within a few days.


In the November 1, 1987, article "Liability in the Air: The Threat of Indoor Air Pollution" published in the ABA Journal, Legionnaires disease is identified as a likely frequent source of indoor air litigation in the future. The author contends that because, at the time, governmental agencies failed to regulate indoor air quality in commercial and residential settings, private lawsuits will proliferate until the issues are resolved. As with other product liability and property liability suits, manufacturers, wholesalers, distributers, property owners, and builders will likely be the targets in such lawsuits. The primary cause of action for these suits will be breach of contract and warranty, breach of implied warranty, and consumer protection claims.

According to Indoor Air Quality: A Guide for Facility Managers, a potential trap for property managers is lowering the water temperature to lower energy costs. This seemingly innocuous action may create a breeding ground for Legionella bacteria. Moreover, some homeowners lower the water temperature on their hot water heaters to save money in heating costs, but this may also result in fostering the growth of Legionella.


Governo Law Firm has represented clients in environmental and toxic tort claims for three decades. We regularly follow scientific, medical and legal developments in toxic tort claims, including Legionnaires disease, asbestos, chemicals, lead and mold. If you have any questions or would like more information, please contact David Governo at [email protected] or Vincent DePalo at [email protected].


Please click here.