What Is Legionnaire's Disease?

Legionnaires’ Disease is a severe form of pneumonia that sickens about 5,000 people in the U.S. each year. It’s caused by the bacterium Legionella pneumophila.

Scientists first identified it in 1977. That was 6 months after a mysterious outbreak sickened 180 and claimed the lives of 29 people attending an American Legion convention at a Philadelphia hotel.


Legionella is usually found in freshwater settings, including lakes, rivers, and streams. Legionella can also survive in soil. But most people don’t get Legionnaires’ there.

Legionella thrives in warm water. It often spreads through a building’s contaminated water system.

It’s actually an airborne disease. The bacterium is so tiny that it can hitch a ride inside tiny water droplets such as mist and water vapor. You can then inhale those droplets, such as in the steam from a sauna or hot tub, and from there the bacterium makes its way into your lungs.

Hot tubs, contaminated air conditioning units, and mist sprayers at grocery stores are prime breeding grounds for legionella if they’re not properly maintained. Legionnaires’ can thrive on cruise ships and in swimming pools and in gyms. Legionella can also multiply in decorative water fountains.

It’s less common, but you can get legionella by drinking tainted water that goes down the “wrong pipe” -- your trachea (which goes to your lungs) instead of your esophagus (which goes from your mouth to your stomach).

Is It Contagious?

No. Legionnaires’ cannot spread from person to person.


How do you know if you have Legionnaires’ Disease? If you’re exposed to legionella, it usually takes 2- 10 days for the disease to take hold.

The disease often seems like the flu. The first symptoms people usually get include headaches, muscle pain, chills, and a high fever that can top 104 F.

By the second or third day, the disease will have fully settled in. You’ll cough and have a hard time breathing. You might have chest pain, nausea, vomiting and diarrhea.

Legionella is also responsible for Pontiac fever, a mild flu-like illness that’s much less severe than Legionnaires’. If left untreated, Pontiac fever will go away on its own. But Legionnaires’ Disease can be life-threatening without treatment.


Who Is Most Likely to Get It?

Even if you are exposed to legionella, there’s a chance that you might not get sick at all. People who are more likely to get sick include those who are:

  • Age 50 or older
  • Former or current smokers
  • People with chronic lung ailments
  • Those with weak immune systems


You might take a simple blood test or get a chest X-ray, although an X-ray will only show how far the infection has spread.

Your doctor might also want you get a CT scan of your brain that takes a series of X-ray images from different angles. And you might get a spinal tap.


When doctors diagnose Legionnaires’, they can treat it quickly with antibiotics.

You may get one of three different types of antibiotics:

  • fluoroquinolones -- Levaquin is preferred, others include moxifloxacin
  • macrolides -- including erythromycin, azithromycin, and clarithromycin
  • tetracycline -- including doxycycline

Your doctor will decide the best treatment option depending on your case.

WebMD Medical Reference Reviewed by Jennifer Robinson, MD on November 25, 2018



CDC: “Legionella (Legionnaires’ Disease and Pontiac Fever)”, “Legionnaires’ Disease and Pontiac Fever. Causes, How it Spreads, and People at Increased Risk.”

Mayo Clinic: “Legionnaires’ Disease. (Symptoms and Causes)”, “Legionnaires’ Disease (Diagnoses and Treatment).”

Legionell.org: “How Legionnaires Disease is Treated.”

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