Legionnaires' disease is a severe form of pneumonia caused by...

Legionnaires' disease is a severe form of pneumonia caused by a type of bacteria called Legionella, according to the CDC. This colorized scanning electron micrograph with moderately-high magnification depicts a large grouping of Gram-negative Legionella pneumophila bacteria. Credit: Wikimedia Commons / CDC

A fourth person has died of Legionnaires' disease, in an outbreak of the treatable disease clustered in the South Bronx that has now sickened at least 65 people, Mayor Bill de Blasio's office said Saturday.

Just a day earlier, the city health department's tally had counted three deaths and 57 infections since July 10, the date the outbreak is thought to have started.

It's the worst outbreak in New York City in two years, de Blasio spokesman Peter Kadushin said.

All four fatalities have been of older adults with pre-existing health problems, such as lung ailments or compromised immune systems, Kadushin said.

The third death was Thursday night. Kadushin said city officials do not want to say when the others died, because doing so could help identify them.

The disease, a treatable type of pneumonia, is transmitted through air conditioning, showers, baths, cooling towers or other water sources. It can't be transmitted directly between people, and the city says its drinking water is safe.

The increase in air-conditioning use on hot summer days, because of how the technology cools the air, quickens the spread of the disease through contaminated mists, which makes for seasonal upsurges. The disease thrives in warm water, which is sometimes used for air conditioning and to cool other utilities.

Symptoms include coughing, shortness of breath, high fever, muscle aches and headaches.

Of the 65 people infected, 55 were hospitalized, and 20 have been discharged, Kadushin said.

He said cooling towers at an additional two buildings were found to be contaminated, bringing the total to five buildings.

Decontamination is ongoing at those buildings, home to Verizon and Streamline Plastic Co.

Epidemiological testing is being performed to see whether any of the 65 cases are linked to the five locations, which also include Lincoln Medical Center as well as a shopping center/movie theater and a hotel.

The city health department said that typically, 5 percent to 10 percent of Legionnaires' disease cases are fatal.

Legionnaires' disease is so named because the first cases were identified at an American Legion convention in 1976 in Philadelphia.

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