In July 1976, thousands of American Legionnaires gathered to celebrate the American bicentennial. Within a few weeks, hundreds of them were seriously ill and 34 had died. All had symptoms of severe pneumonia, some with fevers topping 107 F. It took nearly six months to identify the cause of the illness, a bacterium that causes lung infection and which became known as Legionnaires' disease.

Since then, sporadic outbreaks of Legionnaires' disease have occurred around the world — in hospitals and nursing homes, hotels, prisons, on cruise ships and even at garden shows. The common denominator is water and occasionally soil contaminated with the bacteria that cause the disease. Most often, people develop Legionnaires' disease after they inhale bacteria now called legionella, which flourish in large air conditioning systems, showers, whirlpool spas and in some cases, potting soil. It's not entirely clear why some people exposed to the bacteria become sick and others don't, although older adults, smokers and people with weakened immune systems are especially vulnerable.

The same bacteria that cause Legionnaires' disease also cause Pontiac fever, a much milder illness resembling the flu. Separately or together, the two illnesses are sometimes called legionellosis. Pontiac fever usually clears on its own in a few days with no lasting effects, but untreated Legionnaires' disease is sometimes fatal. Although prompt treatment with antibiotics can usually cure Legionnaires' disease, some people continue to experience problems even after treatment.

Legionnaires' disease usually develops two to 14 days after exposure to the legionella bacteria. It frequently begins with a headache, muscle pain, chills and a fever that may be 104 F or higher. By the second or third day, most people develop other signs and symptoms, including:

• Cough, which may bring up mucus and sometimes blood
• Shortness of breath
• Chest pain
• Fatigue
• Loss of appetite
• 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.

Pontiac fever is a mild form of Legionnaire's disease. It's marked by fever, chills, headache and muscle aches, but it doesn't infect the lungs. Signs and symptoms usually clear in two to five days.

Unlike Legionnaire's disease, Pontiac fever has a short incubation period — usually two to three days — and affects people with no known risk factors for the illness. Some researchers speculate that Pontiac fever isn't an infection at all, but rather a hypersensitivity reaction to legionella bacteria or to the single-celled organisms (protozoa) in which they replicate.

The bacterium Legionella pneumophila is responsible for most cases of Legionnaire's disease, although several other species of legionella also can cause infection. Legionella bacteria thrive in warm, damp environments. In nature, they can survive for months in lakes, rivers, hot springs and soil. But the levels of bacteria in the natural environment are so low that they rarely cause problems. The real threat is indoors, where Legionella bacteria can multiply in water systems, whirlpool spas, air conditioning systems and even the misters in grocery store produce departments.

Like many microorganisms, legionella bacteria have the ability to anchor themselves to the interiors of pipes, faucets and showerheads. Once attached, the bacteria replicate in a sticky substance called a biofilm. As water flows past, it dislodges some of the biofilm, dispersing bacteria throughout the water system. Although it's possible to contract Legionnaires' disease from home plumbing systems, most outbreaks have occurred in large buildings, perhaps because more-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. The 1976 infection in Philadelphia was traced to the hotel's water system. Subsequent outbreaks have been linked to a range of sources, including whirlpool spas on cruise ships, cooling towers, decorative fountains, swimming pools, physical therapy equipment, and water systems in hotels, hospitals and nursing homes.

Scientists aren't certain how much exposure to the bacteria is needed to cause disease, but some people have developed infections after inhaling contaminated droplets for just a few minutes. And unlike many bacteria, which spread within a small radius, legionella bacteria may be capable of traveling as far as four miles through the air.

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 the disease. Drinking water, even if contaminated, doesn't appear to cause illness.
• Soil. A few people have contracted Legionnaire's disease after working in the garden or using contaminated potting soil. It's also possible that the disease may spread when earth containing the bacteria is stirred up at large construction sites.

Beating the immune system
Once legionella bacteria enter the lungs, they are surrounded by immune system cells (alveolar macrophages) that normally attack and destroy foreign organisms. But rather than being destroyed by these cells, the legionella bacteria actually turn them to their advantage. The legionella bacteria enter the macrophages, use them to grow and replicate, and eventually kill them. When the macrophages die, thousands of new bacteria are released into the lungs, worsening symptoms during the first week of the infection and perpetuating the cycle of the disease.

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 65 years of age or older.
• Abuse alcohol.
• Have a job maintaining the cooling towers in air conditioning systems.

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

See your doctor if you think you've been exposed to legionella bacteria. Be sure to mention any trips you've taken in the past two weeks and where you stayed. Diagnosing and treating Legionnaires' disease as soon as possible can help shorten the recovery period and prevent serious complications. For people at high risk, prompt treatment can mean the difference between life and death.

Legionnaires' disease is underreported and underdiagnosed, primarily because special tests are needed to distinguish Legionnaires' disease from other types of pneumonia. To help identify the presence of legionella bacteria quickly, your doctor may use a test that checks your urine for legionella antigens — foreign substances that trigger an immune system response. You may also have one or more of the following:

• Blood tests
• A chest X-ray, which doesn't confirm Legionnaires' disease but does show the extent of infection in the lungs
• Tests on a sample of your sputum or lung tissue
• A CT scan of your brain or a spinal tap (lumbar puncture) if you have neurological symptoms such as confusion or trouble concentrating

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 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 — eliminate excess fluid and waste material from your blood. When your kidneys lose their filtering ability, dangerous levels of fluid and waste accumulate in your body.

When not treated effectively and promptly, Legionnaires' disease may be fatal, especially in anyone whose immune system is weakened by disease or medications.

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

Outbreaks of Legionnaire's disease are preventable, but it requires meticulous cleaning and disinfection of water systems, pools and spas.

On a personal level, avoiding smoking is the single most important thing you can do to lower your risk of infection. Smoking increases the chances that you'll develop Legionnaires' disease if you're exposed to legionella bacteria.