Legionnaires' disease: Causes, symptoms and treatment

A crowd of people walking in a large building.
One of the most common causes of Legionnaires' disease is breathing in tiny droplets of contaminated water expelled from the ventilation systems of large buildings, such as conference centers or hotels. (Image credit: Shutterstock)

Legionnaires' disease is a severe form of pneumonia caused by bacteria in the Legionella genus. 

When Legionella bacteria infect the lungs, the bugs can cause two different illnesses: a milder infection known as Pontiac fever (because the condition was first described in Pontiac, Michigan, in 1968), and the more serious Legionnaires' disease, which can lead to hospitalization and kills about 1 in 10 people infected, according to the Centers for Disease Control and Prevention (CDC)

The lung infection was first described in 1976 when 182 people became sick with pneumonia and 29 of them died after attending an American Legion convention in Philadelphia — hence the name, Legionnaires' disease. 

Related: More than 100 people got Legionnaires' at a state fair. Hot tubs may be to blame.

Eventually, the outbreak was traced to a new bacterial strain found in contaminated water in the hotel's air conditioning system. Scientists named the new strain Legionella pneumophila.

Legionella bacteria typically live in rivers, lakes and soil, where they rarely sicken people. But once bacteria get inside a building's plumbing system, drinking water distribution systems and cooling towers, they can multiply rapidly in warm, stagnant water. Corroded pipes and a lack of chemical disinfection in public water supplies can also promote bacterial growth, according to a 2019 report from the National Academies of Sciences, Engineering, and Medicine

"There can be Legionella bacteria in the water system and no one can get sick, and it's not clear what shifts the balance toward illness," said Dr. Loreen Herwaldt, an infectious disease specialist at the University of Iowa Hospitals and Clinics in Iowa City. The chance of becoming sick can be affected by the amount of bacteria a person is exposed to, and the risk of infection is higher for people who are older, are smokers or have some underlying health problem, she said. 

What causes Legionnaires' disease? 

Legionellosis is an umbrella term for the two illnesses caused by Legionella bacteria. Besides Legionnaires' disease, the same bacteria also cause Pontiac fever, a milder illness with flu-like symptoms that does not cause pneumonia.

People typically become infected with Legionnaires' disease by either swallowing contaminated drinking water or inhaling, through the nose or mouth, tiny droplets of water mist containing the bacteria, Herwaldt said. 

Water may become contaminated by passing through poorly maintained water and ventilation systems in large buildings, such as offices, hotels and hospitals, or on cruise ships. According to the CDC, the most likely sources of bacterial exposure include water mist from the following sources: 

  • Cooling towers (structures containing water that are part of air conditioning systems) and evaporative condensers
  • Hot water tanks and heaters
  • Hot tubs
  • Decorative fountains
  • Showerheads and sink faucets
  • Supermarket produce misters
  • Humidifiers

Air conditioning units in homes and cars are not at risk for Legionella growth because they don't use water to cool the air, the CDC states.

One way that people can become sick is by aspirating contaminated drinking water. This happens when water containing Legionella accidentally enters the lungs — usually because of choking or coughing while drinking — rather than going into the esophagus and stomach

The mechanism to prevent aspiration may be defective in people who smoke or have lung disease — two groups more likely to develop Legionnaires' disease, according to the website Legionella.org

Related: Disney Legionnaires' outbreak: Why cooling towers spread the disease

Disease outbreaks are relatively rare but occur more frequently in summer and early fall, when cooling towers typically run. In 2018, about 10,000 cases of Legionnaires' disease were reported in the United States, according to the CDC. But the number of actual infections may be higher because many cases are not diagnosed or reported. 

In fact, the 2019 report from the National Academies of Sciences, Engineering, and Medicine estimated that between 52,000 and 70,000 Americans develop Legionnaires' disease every year. The report also suggests a more than fivefold increase in reported cases in the country between 2000 and 2017, likely due to an aging population with more chronic diseases as well as more people living in cities with older water systems. 

Risk factors

Most people exposed to Legionella bacteria do not get sick; only 5% of people exposed contract Legionnaires' disease. According to the Mayo Clinic, people at increased risk of developing Legionnaires' disease include:

  • Adults ages 50 and older.
  • Current or former cigarette smokers.
  • People with chronic lung disease, such as emphysema or chronic obstructive pulmonary disease (COPD).
  • People with weak immune systems from diseases such as HIV/AIDS, cancer, diabetes or kidney failure, or people who take medications that weaken the immune system, such as chemotherapy or transplant recipients.

Legionnaires' disease and Pontiac fever cannot be spread from one person to another.


Symptoms of Legionnaires' disease usually begin two to 10 days after being exposed to the bacteria, according to the CDC. The first symptoms may resemble the flu and include a high fever (103 degrees Fahrenheit, 39 degrees Celsius, or higher). But after several days, symptoms seem similar to other types of severe pneumonia, resulting in coughing and difficulty breathing that may require hospitalization. Those symptoms include: 

  • Cough (dry at first, and later producing phlegm)
  • Shortness of breath
  • High fever and shaking chills
  • Chest pain
  • Muscle aches
  • Headache
  • Confusion
  • Diarrhea, nausea, vomiting and stomach pain

Pontiac fever produces flu-like symptoms that show up a day or two after exposure to Legionella bacteria. Symptoms of this milder illness usually last less than a week, and include:

  • Fever
  • Headache
  • Fatigue
  • Loss of appetite
  • Muscle aches and joint pain
  • Chills 
  • Nausea
  • Dry cough

Diagnosis and treatment

People with Pontiac fever usually recover in a few days without needing treatment and the illness often goes undiagnosed. Cases that do get reported to the CDC are usually discovered during a confirmed outbreak of Legionnaires' disease. The milder illness is detected by a urine or blood test in people who may have been exposed to Legionella bacteria at the same time or place as the outbreak occurred. 

A urine test is usually the best way to diagnose Legionnaires' disease, but the test can miss some strains of Legionella bacteria, Herwaldt told Live Science. 

Other tests that may be done include a chest X-ray to detect pneumonia, blood tests and a lab analysis of lung secretions (sputum) to identify the strain of bacteria causing illness. 

Antibiotics are the only treatment for Legionnaires' disease, Herwaldt said. Early treatment with antibiotics can kill the bacteria that cause the lung infection and reduce the severity of the illness. If left untreated, Legionnaires' disease can lead to lung failure and be fatal. 

Additional resources:

This article is for informational purposes only, and is not meant to offer medical advice. 

Live Science Contributor

Cari Nierenberg has been writing about health and wellness topics for online news outlets and print publications for more than two decades. Her work has been published by Live Science, The Washington Post, WebMD, Scientific American, among others. She has a Bachelor of Science degree in nutrition from Cornell University and a Master of Science degree in Nutrition and Communication from Boston University.