Investigators have not been able to identify a specific source of the Legionella bacteria responsible for the Legionnaires disease outbreak in New York City’s Upper East Side, and the outbreak has grown to 36 confirmed cases, according to the NYC Health Department on Wednesday.
Over half of the people infected have had to be hospitalized. There have been no reported fatalities yet, but officials say the number of cases will likely increase as more of them are tested.
The outbreak is localized to the Carnegie Hill and Yorkville areas of the Upper East Side, within ZIP codes 10028, 10128, and 10075. The active testing area is between East 74th Street and East 97th Street. Investigators from the city started their investigation of the cluster on July 2 when two cases of Legionnaires’ disease were reported, possibly from a community source of infection.
Anyone who has lived or worked in the impacted area since the end of June and has symptoms should seek medical attention right away, the city’s mayor, Zohran Mamdani, advises the public. He also said the city will publish the addresses of buildings with positive initial Legionella test results (which has not occurred in past outbreaks).
More than 160 water towers in the area are now being looked at, and some are already being cleaned. Cooling towers are considered the most likely cause of Legionnaires disease outbreaks in NYC; they have historically been the main source of the disease. The exact building(s) containing the source has not yet been determined.
City Council Speaker Julie Menin voiced her concerns Wednesday about whether the Health Department is trying to investigate buildings that didn’t meet a new 2024 law requiring testing and cleaning of cooling towers every 90 days. This law came into effect in May of 2026.
The outbreak has led to comparisons with a fatal outbreak of Legionnaires disease in Harlem during the summer of 2025, which was linked to a cooling tower at a large residential building and ultimately killed seven people.
Health officials said the number of cases could increase further before the outbreak slows, as the water tower remediation continues in the area, but that it would take several days for Legionella to cause illness after exposure.
The NYC Health Department’s emergency response to the Upper East Side Legionnaires disease cluster occurred one day ahead of the Mamdani administration’s Wednesday announcement of further containment measures and promise to provide more transparency of building-level test results.
Earlier this week, residents in the three ZIP codes filled out a community town hall for answers. City health officials announced that the investigation is ongoing and the source of the outbreak has not been identified.
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Source: NYC’s Legionnaires’ disease outbreak continues to grow. What to know
