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

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

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

• Cough, which might 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 — can produce fever, chills, headache and muscle aches. Pontiac fever doesn't infect your lungs, and symptoms usually clear within two to five days.

See your doctor if you think you've been exposed to legionella bacteria. Diagnosing and treating Legionnaires' disease as soon as possible can help shorten the recovery period and prevent serious complications. For people at high risk, such as smokers or older adults, prompt treatment is critical.

The bacterium Legionella pneumophila is responsible for most cases of Legionnaires' disease. Outdoors, legionella bacteria survive in soil and water, but rarely cause infections. However, legionella bacteria can multiply in water systems made by humans, such as air conditioners.

Although it's possible to get Legionnaires' disease from home plumbing, most outbreaks have occurred in large buildings, perhaps because complex systems allow the bacteria to grow and spread more easily. Also, home and car air conditioning units don't use water for cooling.

How the infection spreads

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

• Hot tubs and whirlpools
• Cooling towers in air conditioning systems
• Hot water tanks and heaters
• Decorative fountains
• Swimming pools
• Birthing pools
• Drinking water

Besides by breathing in 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 can develop Legionnaires' disease.
• Soil. A few people have contracted Legionnaires' disease after working in a 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. This can be 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 or other serious condition. This includes emphysema, diabetes, kidney disease or cancer.
• Are 50 years of age or older.

Legionnaires' disease can be a problem in hospitals and nursing homes, where germs can 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 can't 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 filter waste from your blood. When your kidneys fail, dangerous levels of fluid and waste accumulate in your body.

When not treated promptly, Legionnaires' disease can be fatal.

Outbreaks of Legionnaires' disease are preventable, but prevention requires water management systems in buildings that ensure that water is monitored and cleaned regularly.

To lower your personal risk, avoid smoking.

Legionnaires' disease is similar to other types of pneumonia. To help identify the presence of legionella bacteria quickly, your doctor might use a test that checks your urine for legionella antigens — foreign substances that trigger an immune system response. Other tests might include:

• Blood and urine tests
• Chest X-ray, which doesn't confirm Legionnaires' disease but can show the extent of infection in your lungs
• Tests on a sample of your sputum or lung tissue

Legionnaires' disease is treated with antibiotics. The sooner therapy is started, the less likely the chance of developing serious complications. In many cases, treatment requires hospitalization. Pontiac fever goes away on its own without treatment and causes no lingering problems.

You're likely to start by seeing your family doctor. In some cases, you might be referred to a doctor who specializes in treating lung disease (pulmonologist) or infectious diseases, or you might be advised to go to an emergency department.

What you can do

Make a list of:

• Key information about your illness, including your symptoms and when they began. Record your temperature.
• Relevant personal information, including recent hospitalizations and whether you've recently traveled and where you stayed.
• All medications, vitamins and other supplements you take, including doses.
• Questions to ask your doctor.

Bring a family member or friend along, if possible, to help you remember the information your doctor provides.

Questions you might ask your doctor include:

• What is likely causing my symptoms?
• What are other possible causes?
• What tests do I need?
• What is the best course of action?
• I have other health conditions. How will this illness affect them?
• Is it possible to avoid hospitalization? If not, how many days will I be hospitalized?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you questions, including:

• Have your symptoms been continuous?
• Have your symptoms been worsening since their onset?
• What, if anything, seems to improve your symptoms?
• What, if anything, appears to worsen your symptoms?

What you can do in the meantime

To avoid making your condition worse, follow these tips:

• Don't smoke or be around smoke.
• Don't drink alcohol.
• Stay out of work or school, and rest as much as you can.
• Drink plenty of fluids.

If you get sicker before you see a doctor, go to an emergency room.