46 New Yorkers Sick as Legionnaires Outbreak Spreads Across Upper East Side

A Growing Public Health Crisis on the Upper East Side

A severe outbreak of Legionnaires' disease — a dangerous form of pneumonia — has taken hold on New York City's Upper East Side, as city health investigators push to find and remedy the source of infection. As of July 9, 46 people have been diagnosed with Legionnaires' disease in connection with this cluster. More than 60% of those cases have required hospitalization, though no one has died.

The investigation began on July 2, when the NYC Health Department identified two confirmed Legionnaires' disease cases in close proximity and immediately launched an investigation in the Upper East Side neighborhoods of Carnegie Hill and Yorkville, including ZIP codes 10028 and 10128. On July 5, the Department expanded the investigation to include ZIP code 10075 after confirming an additional case involving a person who lives, works, or has visited that area. The case count has climbed steadily ever since.

The Culprit: Rooftop Cooling Towers

The likely source of the bacteria causing this pneumonia is a cooling tower or towers in the area — water systems usually found on the top of buildings that control the temperature of systems such as refrigeration. Legionella typically develops in cooling towers that haven't been cleaned frequently enough. Once the bacteria forms, it drifts into the air through water vapor and cannot be transmitted person-to-person.

As part of its investigation, the NYC Health Department has collected water samples from more than 180 cooling towers. As of July 9, PCR testing found 31 buildings with positive results for the presence of the type of Legionella bacteria that causes Legionnaires' disease. The city is requiring immediate cleaning and disinfection of all cooling towers that receive a positive PCR screening result — a more aggressive strategy than waiting for culture testing to confirm the presence of live Legionella bacteria, a process that takes two weeks.

Mayor Mamdani directed city agencies to respond to this community cluster with urgency and transparency, and for the first time, the administration is releasing the addresses of buildings whose cooling towers tested positive during initial PCR screening. The Upper East Side has a high concentration of cooling towers — more than three times the number tested during last year's Harlem investigation.

Who Is Most at Risk — and What to Watch For

Symptoms typically develop two to fourteen days after exposure and include fever, chills, muscle aches, shortness of breath, and cough. Legionnaires' disease can be effectively treated with antibiotics when diagnosed early. People are at an increased risk for Legionnaires' disease if they are age 50 or older, smoke or vape, have a chronic lung disease, or have a weakened immune system.

Health officials say people who live in those ZIP codes can drink tap water, bathe, shower, and use air conditioners as they normally would. Mist from a cooling tower blows outside and does not affect the internal air conditioning, window air conditioning units, or plumbing system. Anyone who lives, works, or has visited the affected area since late June and develops flu-like symptoms should contact a health care provider immediately.

Lessons From Last Year — and What Comes Next

Over the summer of 2025, seven people died during a Legionnaires' cluster in Harlem. In response, New York City Council changed the law mandating buildings test, clean, and report back to the Health Department every 90 days — but the law has only been in effect since May. The question now is whether that new regulation is being followed, and whether non-compliant buildings are among those fueling the current outbreak.

PCR testing is ongoing at the Public Health Laboratory, and additional buildings may be added to the list of positive results in the coming days. The NYC Health Department has scheduled a public Zoom update on the Legionnaires' disease cluster investigation for July 13, 2026, at 6 p.m. With case counts still rising and dozens of cooling towers yet to be fully cleared, the pressure on city health officials to identify the primary source — and stop it — is only intensifying.