Legionnaires' disease usually develops two to 10 days after exposure to legionella bacteria. It frequently begins with the following signs and symptoms:

• Headache
• Muscle pain
• Chills
• Fever that may be 104 F (40 C) or higher

By the second or third day, you'll develop other signs and symptoms that may include:

• Cough, which may bring up mucus and sometimes blood
• Shortness of breath
• Chest pain
• Gastrointestinal symptoms, such as nausea, vomiting and diarrhea
• Confusion or other mental changes

Although legionnaires' disease primarily affects the lungs, it occasionally can cause infections in wounds and in other parts of the body, including the heart.

A mild form of legionnaires' disease — known as Pontiac fever — may produce signs and symptoms including a fever, chills, headache and muscle aches. Pontiac fever doesn't infect your lungs, and symptoms usually clear within two to five days.

The bacterium Legionella pneumophila is responsible for most cases of legionnaires' disease. Outdoors, legionella bacteria survive in soil and water, but rarely cause infections. Indoors, though, legionella bacteria can multiply in all kinds of water systems — hot tubs, air conditioners and mist sprayers in grocery store produce departments.

Although it's possible to contract legionnaires' disease from home plumbing systems, most outbreaks have occurred in large buildings, perhaps because complex systems allow the bacteria to grow and spread more easily.

How the infection spreads

Most people become infected when they inhale microscopic water droplets containing legionella bacteria. This might be the spray from a shower, faucet or whirlpool, or water dispersed through the ventilation system in a large building. Outbreaks have been linked to a range of sources, including:

• Hot tubs and whirlpools on cruise ships
• Grocery store mist machines
• Cooling towers in air conditioning systems
• Decorative fountains
• Swimming pools
• Physical therapy equipment
• Water systems in hotels, hospitals and nursing homes

Although legionella bacteria primarily spread through aerosolized water droplets, the infection can be transmitted in other ways, including:

• Aspiration. This occurs when liquids accidentally enter your lungs, usually because you cough or choke while drinking. If you aspirate water containing legionella bacteria, you may develop legionnaires' disease.
• Soil. A few people have contracted legionnaires' disease after working in the garden or using contaminated potting soil.

Not everyone exposed to legionella bacteria becomes sick. You're more likely to develop the infection if you:

• Smoke. Smoking damages the lungs, making you more susceptible to all types of lung infections.
• Have a weakened immune system as a result of HIV/AIDS or certain medications, especially corticosteroids and drugs taken to prevent organ rejection after a transplant.
• Have a chronic lung disease such as emphysema or another serious condition such as diabetes, kidney disease or cancer.
• Are 50 years of age or older.

Legionnaires' disease is a sporadic and local problem in hospitals and nursing homes, where germs may spread easily and people are vulnerable to infection.

Legionnaires' disease can lead to a number of life-threatening complications, including:

• Respiratory failure. This occurs when the lungs are no longer able to provide the body with enough oxygen or can't remove enough carbon dioxide from the blood.
• Septic shock. This occurs when a severe, sudden drop in blood pressure reduces blood flow to vital organs, especially to the kidneys and brain. The heart tries to compensate by increasing the volume of blood pumped, but the extra workload eventually weakens the heart and reduces blood flow even further.
• Acute kidney failure. This is the sudden loss of your kidneys' ability to perform their main function — filtering waste material from your blood. When your kidneys fail, dangerous levels of fluid and waste accumulate in your body.