Legionnaires' disease is an infection of the lungs (pneumonia)
caused by a type of bacteria called Legionella pneumophila.
People develop Legionnaires' disease from breathing in the
bacteria, which may be carried in airborne water droplets from contaminated
water sources, such as air-conditioning cooling towers, plumbing systems, hot
water tanks, and spas. These sources often have warm, stagnant water that
allows bacteria to grow. People who work near natural bodies of water do not
appear to be at risk.
Legionnaires' disease is not passed from one person to another. It
usually occurs in single, isolated cases or in groups of people who are exposed
to the same source of infection. Most cases of Legionnaires' disease affect
people older than 50 or people whose immune systems are weakened by other
diseases or by smoking.