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Just when we thought we were finished hearing about Legionnaires' disease, a new cluster has emerged in the Bronx, killing one person and affecting 12 others. This comes right on the heels of the largest Legionnaires' disease outbreak in New York City history in July and August 2015, which killed 12 and sickened 120 others.

Each year, between 8,000 and 18,000 people are hospitalized with Legionnaire's disease in the United States, according to the Centers for Disease Control and Prevention (CDC). But what is it, exactly? Let's get to the bottom of this disease and how you can stay safe.

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1. What Is Legionnaires' Disease?

Legionnaire's disease is a severe form of pneumonia that occurs when bacteria called Legionella pneumophila get into the lungs. There are a couple types of what's called legionellosis, including a milder infection called Pontiac fever, but Legionnaires' disease is the most common form.

Legionella bacteria thrive in stagnant, warm water (from about 80 to 158 degrees F), according to the World Health Organization, which is why people usually get Legionnaires' by inhaling mist or vapor that contains the bacteria. Outbreaks are most common in the summer and early fall, when cooling towers in buildings are turned on again after some period of being turned off and the water has not been treated.

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Legionella can also be found in hot tubs, hot water tanks and decorative fountains, but it's most often found in large buildings.

2. What Are the Symptoms of Legionnaires' Disease?

Legionnaires' symptoms are similar to those of pneumonia. People infected with Legionella bacteria may experience symptoms two to 10 days after exposure to the bacteria, which include:

  • Shortness of breath
  • Cough
  • High fever
  • Muscle aches
  • Headaches

"People think that there's a unique aspect to it and a unique story, but in many cases it's [someone who] just came in [with] something that appeared to be run-of-the-mill pneumonia," says Atul Malhotra, MD, chief of pulmonary and critical care at University of California, San Diego's School of Medicine.

3. Who Is at Risk of Contracting Legionnaires' Disease?

Although the disease can affect anyone, most healthy people who are exposed to Legionella bacteria will not become infected. It most often affects people over age 50 and rarely affects people younger than 20.

The most common risk factor for developing Legionnaires' is heavy cigarette smoking. Other people at a higher risk of developing serious Legionnaires' symptoms include: those with compromised immune systems, such as organ transplant patients; patients taking corticosteroid medicines; and people with pre-existing breathing problems, especially chronic lung disease.

4. How Deadly Is Legionnaire's Disease?

Between 5 and 30 percent of cases of Legionnaire's disease may result in death, according to the CDC, but it's important to note that the disease is not contagious and most cases can be treated successfully with antibiotics.

5. How Can We Prevent Future Legionnaires' Outbreaks?

Legionella is naturally present in the environment, and most buildings have Legionella to a certain extent — it just comes down to proper building maintenance to keep the bacteria in check, says Josh Sharfstein, MD, associate dean of public health practice and training at the School of Public Health at Johns Hopkins University.

"It's a risk in a lot of buildings, but very few have an outbreak," Dr. Sharfstein says. "You have to be very vigilant with the water supply and you can set up a system, but if people aren't doing the testing correctly or some of the machinery starts to break down, you wind up with little pockets of stagnant water."

But once Legionella has taken hold of a water system, the problem can be tricky to fix. Often, it can take days or weeks to fully clear a system of Legionella. Treatment of a system may include eliminating stagnant water, chlorinating the water and having building residents use only bottled water until the system is cleared.

Unfortunately, the average person living in a large building doesn't have much control over Legionella prevention, Sharfstein says, except to properly disinfect your hot tub or decorative fountain if you own one, and to hold public health departments accountable.

"There's really nothing an individual can do to stay safe, you need the building water to be safe, and for that you need a public health department," he says. "There's a whole public health department behind the scenes, and when you see problems like this, it makes you appreciate it a little more."

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