Definition of Legionnaire's disease:

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

Causes, incidence, and risk factors:

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

Spread of the bacteria between humans has not been proven.

Most infection occurs in middle-aged or older people, although it has been reported in children. Typically, the disease is less severe in children.

Risk factors include:

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