Definition of Legionnaire's disease:

Legionnaire's disease is an acute respiratory infection caused by Legionella bacteria.

Causes, incidence, and risk factors:

The bacteria that cause Legionnaire's disease have been found in water delivery systems. They can survive in the warm, moist, air conditioning systems of large buildings, including hospitals.

Most cases are caused by Legionella pneumophila. The rest of the cases are caused by other Legionella species.

Spread of the bacteria from person to person has not been proven.

Most infections occur in middle-aged or older people, although they have been reported in children. Typically, the disease is less severe in children.

Risk factors include:

• Alcoholism
• Cigarette smoking
• Diseases such as kidney failure or diabetes
• Diseases that weaken the immune system, including cancer
• Long-term (chronic) lung disease, such as chronic obstructive pulmonary disease (COPD)
• Long-term use of a breathing machine (ventilator)
• Medicines that suppress the immune system, including chemotherapy and steroid medications
• Older age