A fourth person has died amid the Legionnaires’ disease outbreak in central Harlem, according to the New York City Department of Health. The outbreak that began in late July has since spread to 99 confirmed cases as of Thursday, with four deaths and 17 hospitalizations. Those who have died have not been identified.

The disease is a form of pneumonia caused by water contaminated with Legionella, which can be found in cooling towers, showers and hot tubs.

There have now been 90 confirmed cases of Legionnaires' disease in central Harlem, and 15 people are currently hospitalized battling the disease, the city's health department announced.