Legionnaire's disease is a mild to severe pneumonia. It is caused by the bacteria Legionella pneumophila. Legionnaires' disease occurs when people are exposed to water contaminated with the bacteria.

What is going on in the body?

The Legionella pneumophila bacteria enter the lungs when the person inhales contaminated water. The bacteria grow in the lungs and cause pneumonia, or lung infection. The bacteria can also cause extrapulmonary infections, or infections outside the lung. Extrapulmonary infections occur when the bacteria spread through the bloodstream or lymph system. The most common site of infection outside of the lungs is the heart.

What are the causes and risks of the infection?

Legionnaires' disease is caused by the Legionella pneumophila bacteria. The bacteria are found in many kinds of water supplies. The bacteria grow rapidly in water that is warm and still. Some of the most common sources of infection are as follows:

Legionnaires' disease was first identified in the United States. It has been detected in several other countries. Scientists believe it occurs everywhere in the world. Following is a list of people who are at higher risk for this infection:

The infection can occur at any time, but it is most common in the summer and early fall. It is more common in humid areas of the U.S. It is less common in the arid West.