New York City's death toll from Legionnaires' disease has risen to seven, an increase from four fatalities reported Sunday, Mayor Bill de Blasio's office said Monday night.

The victims -- all older adults with pre-existing medical conditions -- were part of an outbreak of the treatable disease that is clustered in the South Bronx, the mayor's office said.

City officials, including Health Department representatives, Bronx Borough President Ruben Diaz Jr. and Public Advocate Letitia James, held a town hall Monday night at the Bronx Museum of the Arts to share information with concerned residents about the "noncontagious and highly treatable disease."

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"This a form of pneumonia that we see. . . . We have 200 or 300 cases every year," said Health Commissioner Dr. Mary T. Bassett at the event. "It's when we start to see these clusters that we begin to become concerned."

The city has moved "swiftly" to decontaminate the source of the outbreak -- a series of cooling towers in the Bronx, she said. "We respond vigorously to these outbreaks and we eliminate the source," she said.

Legionnaires' disease is believed to have broken out in the area on July 10.

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It thrives in warm water and can be transmitted through air conditioners, showers, cooling towers and other water sources. But the mayor's office stressed that the city's water supply, water towers and home air conditioners are safe from legionella.

The three latest victims died in recent days and were reported to the Health Department on Sunday, the mayor's office said.

The disease has sickened 81 people, up from 71 on Sunday, the mayor's office said. It has led to 64 people being hospitalized, with 28 of those treated and released.

Seventeen cooling towers have been tested and five were positive for Legionnaires' disease, the mayor's office said. The five sites have been decontaminated, but all the locations must submit by Friday their long-term plans to protect against any future growth of legionella, the mayor's office said.