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What Is Legionnaires’ Disease?
Older adults are at greater risk for developing this type of pneumonia
An outbreak of Legionnaires' disease in New York City is caused by the bacteria Legionella pneumophila.
James Cavallini/Science Source
Key takeaways
- Legionnaires’ disease is a serious pneumonia caused by Legionella bacteria that can grow in contaminated water systems.
- Older adults face the highest risk of severe illness from the infection.
- Early testing and antibiotic treatment are important because symptoms can resemble flu, pneumonia or other illnesses.
A recent outbreak of Legionnaires’ disease in New York City has infected more than 70 people and killed four, raising questions about how the infection spreads and who is most at risk.
The good news: Legionnaires’ disease, which is a serious form of pneumonia caused by Legionella bacterium, is treatable. But older adults and people with certain health conditions are more vulnerable, making it important to know the symptoms and when to seek care.
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Here’s what you need to know.
What is Legionnaires’ disease?
Legionnaires’ disease is a type of pneumonia that’s caused by Legionella bacterium, says Dr. Jin Suh, chief of infectious disease at St. Joseph’s Health in Paterson, New Jersey.
Pontiac fever, a less serious illness caused by the same bacteria, can lead to flu-like symptoms such as fever or muscle ache.
In rare cases, Legionella can infect other parts of your body outside of the lungs, such as the heart and brain, or by way of skin infections, Suh says.
What causes Legionnaires’ disease?
Legionella bacterium naturally occurs in lakes, streams and soil. It’s an environmental pathogen, Suh says. However, there’s typically not enough bacteria in the natural environment to cause illness.
In New York City, where several hundred people on average become ill from it each year, cases are more common in the summer. That’s when municipal water enters buildings’ rooftop cooling towers, which are part of their air-conditioning and refrigeration systems. That water can release bacteria-contaminated mist into the air. Dr. Mohamed Hamdy Yassin, an infectious disease specialist with the University of Pittsburgh Medical Center, says doctors call Legionnaires’ disease “summer pneumonia” because outbreaks often happen in warm weather. The infection is more common between June and October.
The bacterium can also grow in sink faucets, shower heads and hot tubs at home. What’s more, you can get Legionnaires’ disease through aspiration, or when something goes down the wrong pipe, but that’s not as common.
Yassin, who sees about 15 to 20 cases of Legionnaires’ disease each year, says the infection doesn’t always come from large buildings. Most cases he has seen come from an issue with a home’s water system. If your home water system has problems or hasn’t run in a while, the bacteria can build up to harmful levels.
“It’s not uncommon to see people living in areas where their water system is not very well maintained,” Yassin says, and those people are at higher risk for the disease.
Another cause of infection is in people who own a second home where the water doesn’t run for months. (In that case, run your water for about 30 minutes before taking a shower or drinking the water, Yassin advises.)
There are at least 60 different strains of Legionella, though most illnesses are caused by Legionella pneumophila, according to the Centers for Disease Control and Prevention (CDC).
What are the symptoms of Legionnaires’ disease?
Legionnaires’ disease causes symptoms that can range from mild to severe. The disease can feel like the flu or pneumonia, or include gastrointestinal or neurological symptoms. Symptoms usually begin two to 14 days after you’re exposed to the bacteria.
Common Legionnaires’ disease symptoms include:
- Fever that often goes above 104 F
- Cough (usually dry)
- Headache
- Shortness of breath
- Nausea
- Diarrhea
- Muscle aches
- Stomach pain
- Confusion
- Coughing up blood
“It’s very tricky,” Yassin says, because symptoms can mimic other conditions. With Legionnaires’ disease, people often feel very sick beyond respiratory symptoms, he adds.
Studies show that 2 to 10 percent of pneumonias that come into a hospital caused by Legionella.
One thing to note: The illness is not spread from person to person; you have to come into direct contact with the bacteria to get it.
How is Legionnaires’ disease diagnosed?
Your health care provider will perform a chest X-ray or a computer tomography (CT) scan to confirm if you have pneumonia.
How Legionnaires’ Got Its Name
The disease is named after an outbreak at an American Legion convention in Philadelphia in July 1976. Of the 182 Legionnaires who contracted the disease, 29 died. The strain of bacteria that causes the disease was later named Legionella.
You need additional tests to determine whether Legionella bacteria are causing the pneumonia. These include urine, blood and phlegm tests. Your doctor may conduct a bronchoscopy, which involves inserting a thin tube to see inside your lungs and collect samples. Or they may conduct thoracentesis, inserting a needle in your chest area to drain fluid from your lungs to check for Legionella.
Hospitals should test everyone with pneumonia for Legionella, but not every doctor does that, Yassin says.
If you don’t feel right, especially if you’re near the area of a confirmed outbreak, seek medical attention, Suh says. “Legionnaires’ is very treatable, especially if you catch it early,” he adds.
How is Legionnaires’ disease treated?
Your doctor will likely prescribe an antibiotic either orally or intravenously. They may give you oxygen if you have trouble breathing; in severe cases, mechanical ventilation may be necessary. Expect to stay in the hospital while you’re being treated.
How many people get Legionnaires’ disease each year?
In recent years, it has affected 8,000 to 18,000 people annually. Between 2016 and 2022, people over the age of 40 contracted more than 9 in every 10 cases. Adults aged 65 and older accounted for about half of all cases in 2022, the CDC reports.
Who’s most at risk for Legionnaires’ disease?
Most younger, healthy people won’t become ill from Legionella. Older adults have a higher risk for Legionnaires’ disease, as do those who:
- Have a weakened immune system
- Have respiratory illnesses like chronic obstructive pulmonary disease (COPD) or emphysema.
- Live in a long-term care facility, including nursing homes
- Have had a recent hospital stay, recent organ transplant or recent surgery with anesthesia
- Smoke or used to smoke cigarettes
What’s the prognosis?
Outcomes can vary if clinicians do not accurately pinpoint the cause of the pneumonia or prescribe the right antibiotic, Yassin says. People can die without timely care, he adds.
About 1 out of every 10 people who contract Legionnaires’ disease will die due to complications from the illness. Among those who get it while staying in a health care facility, about 1 in 4 will die, the CDC reports.
In some cases, you may experience lingering symptoms after you’re discharged from the hospital. These can include muscle aches, trouble walking, issues with balance and coordination, slurred speech and fatigue.
Those residual effects may take time to heal, and they don’t necessarily mean patients need more antibiotics, Suh says.
“There’s a lot of variability [with Legionnaires’ disease infections],” Suh says, adding that aftereffects aren’t just in older adults.
The key takeaways were created with the assistance of generative AI. An AARP editor reviewed and refined the content for accuracy and clarity.
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