An outbreak of Legionnaires’ disease in a New York City borough has killed 12 people and sickened more than 100 people since mid-July, according to city’s Department of Health and Mental Hygiene. Officials are focusing on rooftop air conditioning units as the possible source of the respiratory illness.

In unrelated cases last month, one man died of Legionnaires’ disease in Chicago and three people in Washington state were also treated for the disease.

“Big outbreaks such as the one in the Bronx are rare,” said Dr. David Priest , an infectious disease specialist at Novant Health Infectious Disease Specialists in Winston-Salem, North Carolina. “When we see cases, they tend to be sporadic.”

“It is an uncommon infection, but we do see several cases a year in Winston-Salem, particularly with immunocompromised patients,” Priest added.

What is Legionnaires’ disease?

Legionnaires’ disease is a type of pneumonia caused by the Legionella bacteria. The Centers for Disease Control and Prevention says Legionella bacteria is found commonly in nature and grows best in warm water environments, including:

  • Hot tubs.
  • Cooling towers.
  • Hot water tanks.
  • Decorative fountains or pools.
  • Plumbing systems.

Car and window air conditioning units are not a source of the bacteria, according to the CDC.

The disease is not transmitted person-to-person. Rather, people become sickened with Legionnaires’ when they breathe in a mist or vapor containing the bacteria.

What are the signs and symptoms?

Because Legionnaires’ disease symptoms are similar to other types of pneumonia, it can be difficult to diagnose. Common symptoms include:

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

Most symptoms appear two to 14 days following exposure.

“We’re highly vigilant about Legionnaires’ disease at Novant Health and always keep it on our radar,” Priest said. “If someone is admitted to the hospital with what seems to be pneumonia, we can do a diagnostic test for Legionella.”

“Sometimes a patient with Legionnaires’ disease will have low sodium, as well as gastroenterology issues,” he said. “Legionnaires’ disease can be hard to distinguish from other causes of community-acquired pneumonia.”

How common is it?

Each year approximately 8,000 to 18,000 Americans are hospitalized   with Legionnaires’ disease, according to the CDC. Most people can recover with antibiotics, but in some cases, the disease is fatal. The death rate from the infection can reach up to 30 percent of cases.

“The fatality rate can be high because the disease occurs in patients who may already have other medical problems,” Priest said. “Common antibiotics are used to treat the infection. Patients will take them from 10 to 14 days, but it may take an individual a while to get back to his or her normal self.”

Who is most vulnerable?

Some groups of people are more likely to get sick. These include:

  • People age 50 or older.
  • Smokers.
  • People with chronic lung disease.
  • Persons with weakened immune systems.

How do I protect myself?

Because the bacteria that causes the disease grows best in warmer waters, the CDC urges people to be cautious about hot water vapor. Hot tubs are a common source of the disease-causing bacteria so the agency recommends testing the chemical levels of the water and properly disinfecting hot tubs.

Priest says prevention is the best way to protect oneself from Legionella. “Keep air conditioning units clean and in good working order,” he added.

If you think you may have been exposed to the bacteria or have pneumonia-like symptoms, Priest recommends seeing your primary care provider.