What Is Legionnaires’ Disease? The Outbreak at Disneyland

Legionnaires’ disease has hit the headlines with an outbreak at Disneyland in California. But what is this mysterious illness—and why the panic?

legionnaire's disease

Each year, between 8,000 and 18,000 people in the U.S. are hospitalized with Legionnaire's disease, according to the Centers for Disease Control and Prevention.

Illustration courtesy of CDC

“Disneyland shuts down cooling towers over Legionnaires’ cases” was the headline on Nov. 12, 2017, at CNN’s website—a news story that quickly hit all the major media outlets. To date, 12 people in the Anaheim, Calif., area were confirmed to have contracted Legionnaires’ disease, a serious and potentially lethal pneumonia. Nine of the 12 had visited Disneyland Park.

Dr. Pamela Hymel, chief medical officer for Walt Disney Parks and Resorts, revealed that two cooling towers were shut down and decontaminated, due to “elevated levels of Legionella bacteria.” Officials believe that there is no ongoing risk.

How Common Is Legionnaires’ Disease?

In the United States, there are an estimated 8,000 to 18,000 cases of Legionnaires disease every year, but most are unreported. Around 80 percent are sporadic (isolated) cases throughout the year, with around 20 percent being outbreaks occurring in the warmer months. In adults, up to 15 percent will require hospitalization; some will end up in the ICU (intensive care unit).

“In the United States, reported cases of Legionnaires’ disease have grown by nearly four and a half times since 2000,” according to the Centers for Disease Control and Prevention (CDC). “It is unclear whether this increase represents artifact (due to increased awareness and testing), increased susceptibility of the population, increased Legionella in the environment, or some combination of factors.”

What Is Legionnaires’ Disease, and How Dangerous Is It?

Legionnaires’ is a bacterial infection that was first recognized and named after an outbreak at an American Legion convention in Philadelphia in 1976. Caused by the bacteria Legionella pneumophila (L pneumophila), it is among a group of related bacteria that is a common cause of community-acquired and hospital-acquired pneumonia.

“Legionellosis” refers to two syndromes caused by L pneumophila: Legionnaires disease and Pontiac fever, a mild flu-like illness.

How dangerous is Legionnaires’ disease? The mortality rate among those who contract it is between 5 and 80 percent, but the average in an outbreak is 10 percent. The very young and the very old, the chronically sick (with lung disease, cancer, HIV), and those who acquired the infection in a hospital are most at risk of death. Mortality is increased when diagnosis and treatment with antibiotics is delayed. If acquired in hospital the death rate can reach as high as one in four.

What Causes Legionnaires’ Disease?

The Legionella pneumophila bacteria lives in aquatic habitats such as reservoirs, freshwater streams and lakes, and water storage systems—for example, cooling towers, humidifiers, whirlpool spas, hot water systems (distributed by faucet or shower head), and drinking water systems. The bacteria live in freshwater amoebae.

Transmission occurs when contaminated water is inhaled (aspired) either in the form of aerosol droplets or water. Rarely, cases of Legionnaires’ disease in newborns have been reported following water births in a hospital. The infection is not transmitted person to person.

The Centers for Disease Control and Prevention advises, “Talk to your doctor or local health department if you believe you were exposed to Legionella, and you develop symptoms, such as fever, cough, chills, or muscle aches. Your local health department can determine whether or not to investigate. Be sure to mention if you spent any nights away from home in the last 10 days.”



As Legionnaires’ presents as a pneumonia or flu like illness, it is important to confirm the cause. Your doctor may order some investigations, including:

  • Blood test. To look at bacteria, white cells, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) and biochemistry.
  • Urine test (urinalysis). In the lab they will look for blood and perform the urinary antigen test, which can help diagnose Legionnaires disease.
  • Sputum culture.
  • Chest x-ray. This will help diagnose pneumonia but will not help ascertain the cause.
  • Bronchoscopy. Rarely, the doctor will need to look into the lungs with a fiberoptic scope and take samples.

Who Is at Risk of Legionnaires’ Disease?

Most people who come in contact with the L pneumophila bacteria will not get ill, and if they do, it will be a vague flu-like illness. Some individuals are at higher risk of becoming unwell.

Risk factors include:

  • Age. Newborns and babies the age of one and the elderly are most at risk, but those over the age of 50 are at risk.
  • Those with chronic medical conditions, including lung disease, HIV and other immune deficiencies, cancers, alcoholism, kidney failure, and diabetes, especially those in hospital. Treatment with steroids also increases risk.
  • Gender. Men are affected twice as often as women.
  • Cigarette smokers.
  • Water births.
  • Recent surgery.

Signs and Symptoms of Legionnaires’ Disease

Most people who contract Legionnaires’ disease present with a pneumonia, two to 10 days after exposure. Symptoms include:

  • High fever
  • Severe fatigue and weakness
  • Muscle aches
  • Cough, dry and nonproductive initially, but may become productive with green sputum and sometimes blood in the sputum (hemoptysis).
  • Shortness of breath and chest pain.
  • Headache
  • Low blood pressure

FYIPontiac fever is a milder version of Legionnaires’ disease that begins suddenly, presents with milder symptoms, and usually lasts less than a week.

More severe neurological signs include: Confusion, uncontrolled body movements (ataxia), agitation, and decreased level of consciousness.

Gut symptoms include: watery diarrhea, nausea, vomiting, and abdominal pain.

Newborns (neonates) may present with a severe and rapidly progressing pneumonia or septicemia (blood infection).

Treatment and Prevention of Legionnaires’ Disease

If the doctor has a high level of suspicion of Legionnaires’ disease, or if the cause is confirmed, antibiotic treatment will be initiated promptly. The drug choice will be determined by lab tests and the individual’s age and condition. Treatment may be needed for five to ten days, longer in those with a weak immune system.

According to the Alliance to Prevent Legionnaires’ Disease, prevention of Legionnaires falls into two categories:

  1. Education of the public, building owners and healthcare professionals on the risks of Legionnaires and how to keep reserves of water free from amoeba and bacteria.
  2. Treatment and filtering of water sources including: Source water (reservoirs, lakes, rivers), public water, building water, and water equipment such as faucets, hot tubs, pools, humidifiers, fountains, pipes, drinking water systems and water cooling systems.

For more information on Legionnaires’ Disease, visit The Centers for Disease Control and Prevention, which offers comprehensive advice on the prevention and treatment of the condition.

If you own a hot tub or pool download this fact sheet, to learn how to keep it clean and reduce your risk of Legionnaires’ disease.

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Leonaura Rhodes, MD

Dr. Leonaura Rhodes is a physician turned author, coach, and freelance medical writer and editor. She has worked for Belvoir Media since 2017 and has authored hundreds of articles on … Read More

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