Legionnaires' disease often starts 2 to 10 days after contact with legionella bacteria. It often begins with the following symptoms:

• Headache.
• Muscle aches.
• Fever that may be 104 Fahrenheit (40 Celsius) or higher.

By the second or third day, other symptoms can start, including:

• Cough, which might bring up mucus and sometimes blood.
• Shortness of breath.
• Chest pain.
• Nausea, vomiting and diarrhea.
• Confusion or other mental changes.

Legionnaires' disease mostly involves the lungs. But it can cause infections in sores and in other parts of the body, including the heart.

A mild form of Legionnaires' disease, called Pontiac fever, can cause fever, chills, headache and muscle aches. Pontiac fever doesn't infect the lungs. Symptoms most often clear in less than a week.

Make a medical appointment if you think you've been around legionella bacteria. Diagnosing and treating Legionnaires' disease as soon as possible can help shorten the time it takes to recover and prevent serious complications. People at high risk, such as people who smoke or older adults, need treatment right away.

The bacterium Legionella pneumophila causes most cases of Legionnaires' disease. Outdoors, legionella bacteria live in soil and water. In those places the bacteria rarely cause infections. But legionella bacteria can grow in water systems made by humans, such as air conditioners.

Although people can get Legionnaires' disease at home, most outbreaks have been in large buildings. That might be because bacteria can grow and spread more easily in large systems. Also, home and car air conditioners don't use water for cooling.

How the infection spreads

Most people become infected when they breathe in tiny water droplets that have legionella bacteria. This might be from the spray from a shower, sink or hot tub. Or it might be from water from the system that cools large buildings.

Outbreaks have been linked to:

• Hot tubs and whirlpools.
• Towers that contain water and a fan, called cooling towers. They're used in air conditioning and other systems in buildings.
• Hot water tanks and heaters.
• Fountains.
• Swimming pools.
• Birthing pools.
• Drinking water systems.

The infection can spread in other ways besides breathing in water droplets. The infection can spread by:

• Aspiration. This occurs when liquids enter the lungs by accident, most often because of coughing or choking while drinking. Aspirating water that has legionella bacteria can cause Legionnaires' disease.
• Soil. A few people have gotten Legionnaires' disease after working in a garden or using potting soil that had the bacteria in it.

Not everyone who comes in contact with legionella bacteria gets sick. People are more likely to get the infection if they:

• Smoke. Smoking damages the lungs. This makes the risk of getting all types of lung infections higher.
• Have a weakened immune system. This can be a result of HIV/AIDS or certain medicines. This includes steroids and medicines taken after a transplant to keep the body from rejecting the organ.
• Have ongoing, called chronic, lung disease or other serious conditions. 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. Those are places where germs spread easily, and people there are at high risk of infection.

Legionnaires' disease can lead to serious complications, including:

• Lung 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, most often the kidneys and brain. As a result, the heart tries to pump harder. But the extra work weakens the heart and reduces blood flow even more.
• Acute kidney failure. This is the kidneys not being able to filter waste from the blood. Acute kidney failure occurs all of a sudden. When kidneys fail, unsafe levels of fluid and waste build up in the body.

When not treated promptly, Legionnaires' disease can kill.

Outbreaks of Legionnaires' disease can be prevented by monitoring and cleaning water systems.

Legionnaires' disease is like other types of pneumonia. To find the legionella bacteria quickly, a healthcare professional might use a test that checks the urine for things that trigger the immune system, called antigens. Other tests might include:

• Blood tests.
• Chest X-ray. This doesn't show the type of infection. But it can show how much infection is in the lungs.
• Tests on a sample of mucus from the lungs, called sputum, or lung tissue.

Antibiotics treat Legionnaires' disease. The sooner therapy is started, the lower the chance of getting serious complications. Often, a hospital stay is needed. Pontiac fever goes away on its own without treatment and causes no other problems.

You're likely to start by seeing your family healthcare professional. You may also see a doctor who specializes in treating lung disease, called a pulmonologist, or a specialist in infectious diseases. Or you might need to get emergency care.

What you can do

Make a list of:

• Key facts about your illness, including your symptoms and when they began. Record your temperature.
• Facts about yourself that might be related. These include having been in a hospital recently or having traveled recently and where you stayed.
• All medicines, vitamins and other supplements you take, including doses.
• Questions to ask your provider.

Bring a family member or friend along, if you can, to help you gather all the facts you get.

Questions you might ask your provider 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?
• Do I have to go to a hospital? If so, for how long?

Be sure to ask all the questions you have about your condition.

What to expect from your doctor

A member of your healthcare team is likely to ask you questions, including:

• Have your symptoms been ongoing?
• Have your symptoms gotten worse?
• What, if anything, seems to improve your symptoms?
• What, if anything, seems to make your symptoms worse?

What you can do in the meantime

To keep from 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 healthcare professional, get emergency care.