Legionnaires' Disease

Medically Reviewed by Carmelita Swiner, MD on December 04, 2022
3 min read

Legionnaires’ disease is a severe form of pneumonia. About 5,000 people in the U.S. get it each year. It’s caused by the bacteria Legionella pneumophila.

Scientists first identified it in 1977, 6 months after a mysterious outbreak sickened 180 people and killed 29 attending an American Legion convention at a Philadelphia hotel.

If you’re exposed to legionella, it usually takes 2 to 10 days for the disease to take hold.

It often feels like the flu at first. You may have:

By the second or third day of your illness, you may have:

Pontiac fever

Legionella also causes a disease called Pontiac fever. A few things set it apart from Legionnaires’:

  • Pontiac fever is a mild flu-like illness that’s much less severe than Legionnaires’. It doesn’t cause lower respiratory symptoms like coughing.
  • Symptoms of Pontiac fever usually begin 24 to 72 hours after you come into contact with the bacteria. Legionnaires’ symptoms can take up to 2 weeks.
  • Pontiac fever will go away on its own without treatment, often within 3 to 5 days. But Legionnaires’ disease can become life-threatening.

Legionella is usually found in freshwater settings, including lakes, rivers, and streams. It 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 breathe in those droplets, such as in the steam from a sauna or hot tub, and from there, the bacteria make their way into your lungs.

Rarely, you can get legionella by drinking contaminated 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).

How does Legionnaires’ disease spread?

Legionella bacteria can grow and spread in water systems including:

  • Showerheads
  • Sink faucets
  • Cooling towers
  • Hot tubs
  • Decorative fountains or water features
  • Hot water tanks or heaters
  • Large plumbing systems

Is it contagious?

No. Legionnaires’ doesn’t spread from person to person.

Even if you come into contact with legionella, you might not get sick. People who are more likely to get sick include those who are:

  • 50 or older
  • Former or current smokers
  • People with long-term lung conditions
  • Those who have weak immune systems

Your doctor may order one or more of these exams:

  • A urine test
  • Tests of material that you cough up from your lungs (sputum)
  • A blood test
  • A chest X-ray
  • CT scan of your brain 
  • A test of your spinal fluid (spinal tap)

Without treatment, Legionnaires’ can cause problems including:

  • Dehydration
  • Failure of your kidneys or other organs
  • Respiratory failure
  • Shock
  • Sepsis
  • Coma

Your exact treatment will depend on your case. In general, doctors can treat Legionnaires quickly with medications. You may get one of three types of antibiotics: