What Causes Legionella Disease (Legionnaires' Disease)?

Legionella are bacteria that live around us in water and soil. Usually they aren’t harmful, but in some cases Legionella can make us sick. When Legionella infects the lungs it causes Legionnaires’ disease, a type of pneumonia. It can also cause Pontiac fever, a milder illness that usually goes away on its own. While pneumonia can sound scary, the good news is that you can treat Legionella disease with antibiotics. 

Keep reading to find out more about the spread, diagnosis, and treatment of Legionella infection. 

What is Legionella bacteria, and how does it spread?

Legionella bacteria include many species that live in water and soil. Some species, like Legionella longbeachae, are in soil, including potting mixes. Another species, Legionella pneumophila, lives in water. This species is the most common cause of illness in humans. 

Legionella live in freshwater, like lakes and streams. But infections are more likely to occur from human-made water sources, including:

• Hot tubs

• Industrial air conditioning systems

• Large plumbing systems

• Hot water tanks

• Fountains and water features

Legionella can grow and multiply in these water sources, making it easier to cause infection. Testing a water source is the only way to know that it’s contaminated with Legionella. Infection starts after someone inhales contaminated water droplets or soil particles in the air. It can also happen when someone who has difficulty swallowing accidentally inhales contaminated water or ice (aspiration).

How do you get Legionnaires' disease?

Legionnaires’ disease happens when Legionella bacteria infect the lungs. The name of the disease comes from a 1976 outbreak at an American Legion in Philadelphia, Pennsylvania. Outbreaks can occur when groups of people come into contact with the bacteria in contaminated water and get infected. 

In 2018, there were about 10,000 cases of Legionnaires’ disease in the U.S. Single cases of Legionnaires’ disease can occur in individuals, and they’re more common than outbreaks. 

Certain people are at higher risk for Legionnaires’ disease, including:

• Adults over age 50

• People with decreased immune systems

• People who smoke or used to smoke

• People with chronic illnesses (like diabetes and kidney or lung disease) or cancer

Considering the bacteria is always around us, infection with Legionella is rare. This means that even if you’re exposed to Legionella, it won’t always make you sick.

What are the symptoms of Legionella disease?

The symptoms of Legionnaires’ disease are similar to those in other types of pneumonia, including:

• Fever

• Fatigue

• Chest discomfort

• Cough

• Shortness of breath

In Legionnaires’ disease these symptoms start 2 to 10 days after you’re exposed to the bacteria.

Legionella can also cause Pontiac fever, but it’s milder than Legionnaires’ disease. Symptoms usually start 1 to 2 days after exposure and include:

• Fever

• Headache

• Body aches

• Nausea and vomiting

• Diarrhea

How do you diagnose Legionella infections?

Your provider may suspect Legionnaires’ disease based on your symptoms and physical examination. With pneumonia, your lungs sound abnormal when your healthcare provider listens with a stethoscope. A chest X-ray can confirm the diagnosis of pneumonia. 

Since Legionnaires’ disease acts like other types of pneumonia, testing can identify Legionella as the specific cause. Lab testing on a sample of sputum (phlegm) or urine can diagnose Legionnaires’ disease. Testing is most important when pneumonia is severe because it helps guide treatment. 

You don’t need testing for Pontiac Fever. This is because it's a mild illness that goes away on its own — without treatment — within about 1 week.

How do you treat Legionnaires' disease?

You can treat Legionnaires’ disease with antibiotics. Once a healthcare provider diagnoses pneumonia, they’ll recommend you start taking antibiotics right away. This is a common treatment for bacterial causes, including Legionella. You can start antibiotics before Legionella testing comes back. 

Once testing confirms Legionella, your provider may recommend treatment with fluoroquinolones or macrolides if you hadn’t already started taking these. These are the preferred classes of antibiotics for this type of pneumonia. Specifically, antibiotics that treat Legionnaires’ disease include Levofloxacin (Levaquin), a fluoroquinolone, and azithromycin (Zithromax), a macrolide.

If you need treatment in the hospital for Legionnaires’ disease, you’ll likely get antibiotics through an intravenous (IV) infusion. Once your symptoms improve, you’ll switch from IV antibiotics to oral antibiotics (in pill form). Treatment lasts at least 5 days, but some cases need longer treatments.

How serious is Legionella disease?

Legionella disease can range from mild to severe. People with underlying medical conditions are more likely to have severe Legionella disease. Some people may need treatment in the hospital. About 1 in 10 people with Legionnaires’ disease will die from it. 

There’s no mistaking that Legionnaires’ disease presents a serious health risk. But it’s important to remember that antibiotics can treat most cases of Legionnaires’ disease. Starting antibiotics early in the illness improves outcomes. So if you notice symptoms, be sure to connect with your healthcare provider right away.

Is Legionella disease contagious?

According to experts, Legionella disease is very unlikely to spread between people. This means to get it you must be exposed to water or soil with Legionella bacteria. There’s one known probable case of Legionella disease that spread from one person to another. But this is very unusual. 

Why trust our experts?

Written by:

Sarah Pozniak, MD

Sarah Pozniak, MD, has 9 years of experience since graduating medical school (6 since completing residency). She is a board-certified general internal medicine physician and has practiced as a primary care physician in both Utah and Washington, D.C. Currently, she cares for patients with chronic illnesses such as diabetes and high blood pressure using telemedicine as part of a virtual care program.

Edited by:

Mandy Armitage, MD

Mandy Armitage, MD, has combined her interests in clinical medicine with her passion for education and content development for many years. She served as medical director for the health technology companies HealthLoop (now Get Well) and Doximity.

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