MARCH 11, 2025 – Legionnaires’ disease, a severe type of pneumonia, was so named in 1976 after a Pennsylvania outbreak of a mystery illness that sickened over 200 and killed 34 in two weeks. The Centers for Disease Control and Prevention identified the cause of the illness to be a particular type of bacteria. Legionella bacteria were discovered in the cooling towers of a hotel that had been used for an American Legion convention.
“The risk of Legionnaires’ disease is lower now that scientists have learned more about its cause and effective prevention tactics,” says Dr. Charles McCannon, a preventive medicine physician working for Defense Health Agency-Public Health in Aberdeen, Maryland. “While the risk is low, cases still occur.”
Outbreaks occur when two or more people are exposed to Legionella in the same place and get sick at about the same time, says McCannon. The last outbreak confirmed by the military was in 2010 at Selfridge Air National Guard Base, Michigan.
Medical monitoring for cases of communicable diseases such as Legionnaires’ disease is a public health measure that prevents larger-scale disease occurrence.
Tracking Cases in the Military
To identify potential outbreaks and prevent additional exposures to the hazardous bacteria, both the CDC and the U.S. military require medical facilities to report cases of Legionnaires’ disease.
Alexis Maule, a senior epidemiologist with the Preventive Medicine Division of DHA-PH at Aberdeen, explains that the name Legionellosis is now used to refer to both Legionnaires’ disease and Pontiac fever, a milder illness without pneumonia.
“Most people with Pontiac fever likely do not seek treatment for their illness,” says Maule. “In addition, most healthy people who are exposed to Legionella bacteria will not become ill with either Legionnaires’ disease or Pontiac fever. That means the extent of exposure may underreported, but we are identifying cases needing treatment at military medical facilities.”
Even with the expanded case definition, the military has not found Legionellosis to be a commonly reported disease. This is generally consistent with CDC findings though there are higher rates in the general population.
“In the past five years, 87 Legionellosis cases have been reported to the military’s medical reporting system, and only half of these were confirmed to be Legionnaires’ disease,” says Maule. “In addition, the disease was most frequently reported in retirees, while less than a third of reported cases were active-duty military personnel.”
Maule explains that this makes sense because age is risk factor for Legionellosis, and many retirees were noted to already have chronic medical conditions unrelated to Legionellosis, making this group more susceptible to any type of bacterial infection.
McCannon and Maule provide answers to questions about Legionellosis:
Frequently Asked Questions
Q. What is Legionellosis?
Legionellosis comprises the illnesses (Legionnaires’ disease, Pontiac fever) caused by the Legionella bacteria. Many species of Legionella bacteria can cause disease, but the presence of the bacteria in the environment or even in a clinical sample (such as urine or blood) does not mean a person is or will become ill with a disease. The majority of disease is caused by Legionella pneumophila, particularly a sub type known as serogroup 1.
Because the bacteria have the potential to cause harm (even if no clinical cases are reported), the presence of Legionella bacteria detected in a facility is considered a hazard to be remediated. Protocols have been established to ensure proper sanitation is achieved. (https://ph.health.mil/PHC Resource Library/ohs-ihf-control-of-legionella-factsheet.pdf)
Q. How does a person get sick with Legionnaires’ disease?
The primary way people are exposed is by breathing in water droplets that carry the Legionella bacteria. Less commonly, Legionella may be transmitted via aspiration of drinking water. Exposed persons who become sick do not transmit the infection to others.
While Legionella bacteria occur naturally in freshwater environments, they primarily cause disease after growing in certain human-made water systems that aerosolize water. Complex air conditioning cooling towers of large structures such as hotels, hospitals, long-term care facilities, and cruise ships are a particular concern. Water systems and air-conditioning units of private homes are generally not exposure sources. Cases have occasionally been attributed to personal hot tubs, whirlpool spas and even decorative fountains.
The CDC reports that since the 1976 outbreak, agencies around the world now have more stringent cleaning and hygiene standards for cooling towers and large-scale air-conditioning systems. But the conditions outside the U.S. may present more of a concern. (https://www.cdc.gov/legionella/infographics/legionella-affects-water-systems.pdf)
Q. What are symptoms of Legionellosis?
Legionellosis symptoms can include fever and other flu-like symptoms such as muscle aches and cough and can begin hours to several days after bacteria are inhaled. Legionnaires’ disease symptoms begin two to 14 days after exposure and can be confirmed when Legionella bacteria infect cells in the air sacs of the lung. This condition can be life-threatening, but it is treatable with antibiotics.
The symptoms of Pontiac fever develop in five hours to a few days; it is a milder illness—without pneumonia—characterized by dry cough or sore throat, fever, chills, fatigue, headache and sore muscles.
Q. How is Legionellosis diagnosed?
Diagnosis is based on clinical findings, such as signs of pneumonia identified with a chest x-ray, and medical laboratory antigen confirmation of the unique type of the bacteria, known as serotype in the patient. If environmental samples are available, they can be compared to the bacteria type found in patient samples. If the bacteria type does not match the type found at an environmental source of concern, other exposure sources are considered.
Q. Are certain people at higher risk of Legionellosis?
Most healthy people who are exposed to Legionella bacteria do not get infected with disease. Certain behaviors such as smoking, recent travel with an overnight stay outside of the home, recent care at a healthcare facility, and hot tub use are risk factors that would be considered if determining a case.
The risk of Legionellosis is higher among persons over 50 and those with chronic lung disease, immune system disorders due to disease or medication, systemic malignancy, or an underlying illness such as diabetes, renal failure, or hepatic failure. The younger, relatively healthy and vaccinated active-duty population has a lower risk for severe disease from Legionella exposure compared to the general population.
Q. Does risk of Legionellosis depend on location or time?
Within the U.S., the CDC has shown a higher number of cases of Legionellosis in the northern and mid-western states. There is some seasonality to the risk of Legionellosis, with higher risk of infection in the summer through fall months because temperate conditions allow for the growth of the bacteria. (https://www.cdc.gov/legionella/php/surveillance/surveillance-report-2018-2019.html)
Q. How is Legionellosis differentiated from other pneumonias?
Pneumonia is a generalized term for inflammation and infection of the air sacs in one or both lungs resulting in fluid in the lungs. Air sacs fill with fluid or pus, causing a productive cough with phlegm, and often fever, chills and difficulty breathing.
Pneumonia can be a symptom of many different diseases and can be caused by a range of organisms, including bacteria (such as Legionnaires’ disease and Q fever), viruses (such as COVID-19 and certain flu viruses), and fungi (such as Coccidioidomycosis [“Valley fever”]). The severity of pneumonia infection can range from mild to life-threatening. Bacterial pneumonia is usually more severe than viral pneumonia.
Pneumonia caused by Legionella bacteria is only differentiated from other pneumonias by medical tests/laboratory tests to determine the cause of the pneumonia.
Q. Does the pneumonia vaccine help against Legionellosis?
There is no vaccine against Legionellosis, and none of the available pneumococcal vaccines protect against the Legionella bacteria. However, Legionnaires’ disease shares many of the same risk factors as other pneumonias, so getting a pneumococcal vaccine, as well as a respiratory syncytial virus, or RSV, vaccine, as recommended, may reduce the risk of severe Legionnaires’ disease.
Additional information sources:
- DHA-PH webpage on Legionella (https://ph.health.mil/topics/discond/diseases/Pages/Legionella.aspx)
- Navy Legionella Fact Sheet (Jan 2021) (https://www.med.navy.mil/Portals/62/Documents/NMFA/NMCPHC/root/Program%20and%20Policy%20Support/Water%20Sanitation%20and%20Safety/Legionella-Fact-Sheet-21Jan2021PDF.pdf)
- Navy Legionella Response Manual (Nov 2020) (https://www.med.navy.mil/Portals/62/Documents/NMFA/NMCPHC/root/Program and Policy Support/Water Sanitation and Safety/NMCPHC_Legionella_Response_Guidance_Man_201120_Rev1_230110.pdf)
- DHA Communicable Disease Toolkit, CDT, guidance for providers – Legionellosis (https://ph.health.mil/topics/healthsurv/de/Pages/cdt.aspx)
- CDT Reference Sheet (https://ph.health.mil/cdt/cphe-cdt-legionellosis-ref.pdf)
- CDT Case Definition Flowchart (https://ph.health.mil/cdt/cphe-cdt-legionellosis-flow.pdf)
- CDT Investigation worksheet (https://ph.health.mil/cdt/cphe-cdt-legionellosis-worksheet.pdf)
- Journal article (2014) “Legionellosis in Military Health System beneficiaries, 1998–2013” (https://pubmed.ncbi.nlm.nih.gov/24684614/)
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