New York City is facing recurrent outbreaks of Legionnaires’ disease, a severe form of pneumonia caused by the Legionella bacteria, which thrives in urban water systems. Over the past month, the city has reported six deaths and 90 cases linked to an outbreak centered on Manhattan’s Upper East Side. This marks the third major outbreak in New York in the past dozen years, following a deadly cluster in Harlem last year and a significant outbreak in the South Bronx in 2015.

First identified in 1976 at a convention of military veterans in Philadelphia, Legionella bacteria multiply in warm, stagnant water, often in environments such as cooling towers, plumbing systems, and hot tubs. New York’s aging infrastructure—characterized by corroded pipes and complex plumbing in large buildings—provides an ideal setting for the bacteria to flourish. The city houses nearly 6,000 cooling towers, mostly concentrated in Manhattan, which have been implicated repeatedly in outbreaks. These towers use warm water and fans to cool air, generating mist that can carry the bacteria across city blocks.

Authorities have responded to recent outbreaks by intensifying testing protocols, requiring building owners to conduct more frequent inspections of cooling towers. After last year’s Harlem cluster, testing intervals were shortened from every 90 days to every 30 days. In the most recent outbreak, the city tested over 180 cooling towers, finding more than 70 positive for Legionella DNA. These included towers atop residential buildings, schools, and cultural institutions such as the Guggenheim Museum and the Metropolitan Museum of Art. In response, the health department mandated immediate disinfection of any towers that tested positive.

Despite these measures, experts emphasize that focusing solely on cooling towers may be insufficient. Research indicates that only about a quarter of Legionnaires’ outbreaks are linked to cooling towers, while over half stem from potable water systems, including building plumbing networks. Legionella forms biofilms inside pipes and fixtures, which can release bacteria when water pressure changes abruptly—such as during fire hydrant use or altered water consumption patterns in large buildings. This presents a risk for sporadic cases beyond large-scale clusters, with residents potentially exposed through routine activities like showering.

Some infectious disease specialists argue that the persistent occurrence of Legionnaires’ in New York signals a need to broaden control efforts beyond rooftop cooling towers to include deeper investigation and maintenance of building plumbing systems. While outbreaks are often traced to mist from cooling towers, single cases frequently arise from bacteria colonizing internal water systems. The city’s health department continues to seek genetic matches between bacteria found in patients and environmental samples to pinpoint sources.

The recent outbreak, which caused several deaths including those of longtime city residents Robert and Christine Goldstein, appears to be contained, with no new cases reported since mid-July. However, officials acknowledge that Legionella remains a pervasive challenge in urban environments with complex water infrastructure. The rise in Legionnaires’ cases nationwide over the past 25 years underscores the ongoing need for vigilance and innovative approaches to managing the bacteria in city water systems.