Four people have died and dozens more have been sickened by a recent outbreak of Legionnaires’ disease in New York City, CNN reports.

“We are concerned about this unusual increase in Legionnaires’ disease cases in the South Bronx,” said Health Commissioner Dr. Mary Bassett in a statement. “We are conducting a swift investigation to determine the source of the outbreak and prevent future cases.”

According to The New York Times, 65 people in the area have contracted the flu-like illness since mid-July and 55 have been hospitalized. From NBC New York:

Legionnaires’ disease is caused by exposure to the bacteria Legionella; in most cases, people are exposed to the bacteria by inhaling contaminated aerosols from cooling towers, hot tubs, showers and faucets or drinking water.

Twenty-two buildings have been visited as “disease detectives” hunt for the source of the outbreak, the city said Friday. Seventeen of those buildings have cooling towers — three of those tested positive for Legionella, including one at Lincoln Hospital; one at Concourse Plaza, a shopping plaza; and one at the Opera House Hotel.

This weekend, Deputy Health Commissioner Dr. Jay Varma warned that it was “very possible” the number of infected could go up in the next seven days.

“We are also quite confident that after that time, the number of cases will go down,” Varma told WABC-TV. “We can never know for sure what the future holds and we are working very aggressively to make sure people at risk—people in senior centers, people at homeless shelters, people with other medical problems—get the message to pay attention to their health.”

Symptoms of the disease include fever, cough, chills and muscle aches.

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