Legionnaires' disease is a severe form of pneumonia — lung inflammation usually caused by infection. Legionnaires' disease is caused by a bacterium known as legionella.
You can't catch Legionnaires' disease from person-to-person contact. Instead, most people get Legionnaires' disease from inhaling the bacteria. Older adults, smokers and people with weakened immune systems are particularly susceptible to Legionnaires' disease.
Legionella bacterium also causes Pontiac fever, a milder illness resembling the flu. Separately or together, the two illnesses are sometimes called legionellosis. Pontiac fever usually clears on its own. But untreated Legionnaires' disease can be fatal. Although prompt treatment with antibiotics usually cures Legionnaires' disease, some people continue to experience problems after treatment.
Legionnaires' disease usually develops two to 14 days after exposure to legionella bacteria. It frequently begins with the following signs and symptoms:
By the second or third day, you'll develop other signs and symptoms that may include:
Although Legionnaires' disease primarily affects the lungs, it occasionally can cause infections in wounds and in other parts of the body, including the heart.
A mild form of Legionnaires' disease — known as Pontiac fever — may produce symptoms including fever, chills, headache and muscle aches. Pontiac fever doesn't infect your lungs, and symptoms usually clear within two to five days.
When to see a doctor
The bacterium Legionella pneumophila is responsible for most cases of Legionnaires' disease. Outdoors, Legionella bacteria survive in soil and water, but rarely cause infections. Indoors, though, Legionella bacteria can multiply in all kinds of water systems — hot tubs, air conditioners and mist sprayers in grocery store produce departments.
Although it's possible to contract Legionnaires' disease from home plumbing systems, most outbreaks have occurred in large buildings, perhaps because complex systems allow the bacteria to grow and spread more easily.
How the infection spreads
Scientists aren't certain how much exposure to the bacteria is needed to cause disease, but some people have developed infections after inhaling contaminated droplets for just a few minutes. And unlike many bacteria, which spread within a small radius, legionella bacteria may be capable of traveling as far as four miles through the air.
Although legionella bacteria primarily spread through aerosolized water droplets, the infection can be transmitted in other ways, including:
Not everyone exposed to legionella bacteria becomes sick. You're more likely to develop the infection if you:
Legionnaires' disease is a sporadic and local problem in hospitals and nursing homes, where germs may spread easily and people are vulnerable to infection.
Legionnaires' disease can lead to a number of life-threatening complications, including:
When not treated effectively and promptly, Legionnaires' disease may be fatal, especially if your immune system is weakened by disease or medications.
Preparing for your appointment
You're likely to start by first seeing your family doctor or a primary care provider. However, in some cases, you may be referred immediately to a doctor who specializes in treating lung disease (pulmonologist) or an infectious diseases doctor, or advised to go to an emergency department.
What you can do
Questions you might ask your doctor include:
In addition to the questions that you've prepared, don't hesitate to ask questions during your appointment any time that you don't understand or need information clarified.
What to expect from your doctor
What you can do in the meantime
Tests and diagnosis
Legionnaires' disease is similar to other types of pneumonia. To help identify the presence of legionella bacteria quickly, your doctor may use a test that checks your urine for legionella antigens — foreign substances that trigger an immune system response. You may also have one or more of the following:
Treatments and drugs
Legionnaires' disease is treated with antibiotics. The sooner therapy is started, the less likely the chance of serious complications or death. Pontiac fever goes away on its own without treatment and causes no lingering problems.
Outbreaks of Legionnaire's disease are preventable, but it requires meticulous cleaning and disinfection of water systems, pools and spas.
Avoiding smoking is the single most important thing you can do to lower your risk of infection. Smoking increases the chances that you'll develop Legionnaires' disease if you're exposed to legionella bacteria.
Last Updated: 2010-12-10
© 1998-2013 Mayo Foundation for Medical Education and Research (MFMER). All rights reserved. A single copy of these materials may be reprinted for noncommercial personal use only. "Mayo," "Mayo Clinic," "MayoClinic.com," "Mayo Clinic Health Information," "Reliable information for a healthier life" and the triple-shield Mayo logo are trademarks of Mayo Foundation for Medical Education and Research.
Terms and conditions of use