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What Is Legionnaires’ Disease?

It’s a type of pneumonia (a lung infection) that can become life-threatening. You can get it after inhaling one of group of bacteria called legionella. There are about 35 types of legionella that scientists link to the disease, but a specific form called L. pneumophila causes 90% of infections.

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Where It Comes From

Just because legionella bacteria happen to be around, it doesn’t mean they’re likely to infect you. In fact, they seem to be mostly harmless in lakes and streams. To cause infection, they usually have to multiply to high levels in human-made systems like showers, hot tubs, air conditioners, fountains, and water towers. 

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How Does It Spread?

You’re unlikely to get this disease from simply drinking or touching contaminated water. Once legionella builds up to a certain level, it must form droplets small enough to move through the air and infect your lungs. Think of steam in a shower or vapor from a humidifier. You also might get it from airborne potting soil, or if you choke on contaminated water and it gets into your lungs (aspiration).

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What Are the Symptoms?

If you get Legionnaires’, you’ll start to feel tired, weak, and achy 2-10 days after you’re infected. It’s common to get a fever higher than 103 F and a cough that can produce colored or bloody mucus. You also may have a headache, shortness of breath, nausea, vomiting, and diarrhea.

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Is It Contagious?

Probably not. There is little, if any, evidence that you can spread Legionnaires’ disease from person to person. So there’s no reason to wear a mask around other people as you might with other serious infections like SARS or influenza.

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Who Gets It?

If you’re healthy and young and you breathe in legionella, you’re less likely than others to get Legionnaire’s disease. Your risk goes up if:

  • You’re over 50
  • You smoke (or have in the past)
  • You have lung disease (such as COPD or emphysema)
  • You have diabetes, cancer, kidney problems, or liver failure
  • Drugs, illness, or an organ transplant has weakened your immune system
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The most common and serious sign of legionella is the pneumonia of Legionnaires’ disease. That’s when pus or fluid fills the sacs in your lungs. Your doctor will listen to your chest, ask about your symptoms, and take a chest X-ray to tell for sure if you have it. A test of your phlegm, spit, or urine can help tell whether legionella or some other germ caused the pneumonia. 

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How Do You Treat It?

Your doctor will prescribe antibiotics.  Two types -- quinolones (ciprofloxacin, levofloxacin) and macrolides (azithromycin) -- work especially well against the bacteria that cause Legionnaires’ disease. The sooner you start, the less likely you are to have complications that put you in the hospital.

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When to See a Doctor

You should see a doctor as soon as possible if you think you’ve been exposed to legionella bacteria. Because it causes such a serious form of pneumonia, early detection and treatment with antibiotics are key to making a full recovery. This is especially true if you’re over 50, you’re already ill, or you’ve been a smoker at some point in your life.

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Lung failure: Your lungs can’t get enough oxygen into your blood or enough carbon dioxide out. 

Septic shock: This can happen if an infection makes your blood pressure drop and cuts blood to your organs. Your heart first works harder to try to help but eventually weakens and cuts blood flow even more.

Kidney failure: When your kidneys stop working, toxins, fluid, and waste build up in your body, which can cause tissue damage and illness.

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Pontiac Disease

You might also hear it called Pontiac fever. Like Legionnaires’ disease, legionella bacteria cause it. But unlike Legionnaires’, it doesn’t cause pneumonia. You usually get flu-like symptoms. Fever, chills, a headache, and muscle aches are common.

It starts about 2 days after you’re infected and usually goes away without treatment in 2-5 days. 

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Why It’s Called That

The name, Legionnaires’ disease, dates back to 1976. That’s when bacteria infected and sickened a group of people attending a Philadelphia convention of the American Legion of Pennsylvania. Scientists named the bacteria Legionella pneumophila and the illness Legionnaires’ disease, after the group that drew attention to this unique form of pneumonia. 

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Sources | Medically Reviewed on 09/30/2019 Reviewed by Jennifer Robinson, MD on September 30, 2019


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Canadian Centre for Occupational Health and Safety: “Legionnaires' Disease.”

CDC: “Legionella (Legionnaires' Disease and Pontiac Fever).”

Health Service Executive (Irish Public Health Service): “Legionnaires’ Disease.” “About the Disease.”

Mayo Clinic: “Legionnaires' disease.”

Merck Manual: “Sepsis.”

United States Department of Labor Occupational Safety and Health Administration: “Legionellosis (Legionnaires' Disease and Pontiac Fever).”

World Health Organization: “Legionnaires' Online Q&A,” “Legionellosis.”

Reviewed by Jennifer Robinson, MD on September 30, 2019

This tool does not provide medical advice. See additional information.

THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for general informational purposes only and does not address individual circumstances. It is not a substitute for professional medical advice, diagnosis or treatment and should not be relied on to make decisions about your health. Never ignore professional medical advice in seeking treatment because of something you have read on the WebMD Site. If you think you may have a medical emergency, immediately call your doctor or dial 911.