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

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)

Symptoms tend to get worse during the first 4 to 6 days. They typically improve in another 4 to 5 days.

Symptoms may include:

• Muscle aches and stiffness
• Joint pain
• Loss of energy
• General discomfort, uneasiness, or ill feeling (malaise)
• Headache
• Fever
• Shaking chills
• Nonproductive cough
• Coughing of blood
• Shortness of breath
• Chest pain
• Diarrhea
• Ataxia (lack of coordination)

The health care provider will perform a physical exam. The doctor may hear abnormal sounds called crackles when listening to the chest with a stethoscope.

Tests that may be done include:

• Urine tests to check for bacteria
• Chest x-ray
• Arterial blood gas
• CBC, including white blood cell count
• Erythrocyte sedimentation rate
• Liver function tests
• Sputum direct fluorescent antibody test