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Legionnaires’ disease, a deadly respiratory illness spread by inhaling contaminated water droplets, is rising again.

Following a pandemic lull, the case rate in 2025 climbed to its second-highest level on record, according to provisional data from the Centers for Disease Control and Prevention.

The U.S. registered 2.9 cases for every 100,000 people, second only to 2018, when the rate was 3.04.

Case rates were highest in the Mid-Atlantic, Midwest, and Southeast. The biggest numbers were in New York City, West Virginia, and Ohio.

Legionella bacteria, the cause of the disease, are found in rivers and lakes, but they multiply in warm, stagnant water inside building plumbing systems, especially on the slimy biofilms that form inside pipes. Legionnaires’ disease is a consequence of the built environment.

The disease’s resurgence last year was marked by a large outbreak in New York City, a center of Legionella risk. The incident in Harlem, in July 2025, sickened 118 people, hospitalizing 92 and killing seven.

The city has witnessed two additional large outbreaks this year. In July, a disease cluster on the Upper East Side resulted in 94 cases and 11 deaths. Then in a September outbreak in the South Bronx, 23 people tested positive, two were hospitalized, and two died.

Cases declined sharply in 2020, probably due to reasons related to the Covid-19 pandemic. There’s no definitive cause but several factors likely contributed, says Kerry Hamilton, an Arizona State University professor who studies pathogens in the built environment. Doctors were distracted and possibly not testing for the disease as frequently. Masks blocked airborne contaminants, including Legionella, which hitches a ride on water droplets. Travel restrictions and hotel closures also kept vulnerable people away from sources of Legionella.

Since the pandemic cases have spiked. The case rate in 2025 is up 50 percent from the pandemic trough, according to data reported to the National Notifiable Diseases Surveillance System.

Legionnaires’ disease case rates, 2000 to 2025. Data and graphic from the Centers for Disease Control and Prevention

First identified in 1976, Legionnaires’ disease is now the deadliest acute waterborne disease in the United States. The disease attacks the lungs and resembles pneumonia, but it is not spread person-to-person. Instead, it is contracted by inhaling water droplets that carry Legionella bacteria.

Anything that creates airborne water droplets – rooftop cooling towers, showers, hot tubs, fountains – can spread Legionella. Older people, smokers, and those with weakened immune systems are most susceptible.

Lead image: New York City had the country’s highest Legionnaires’ disease case rate in 2025, according to Centers for Disease Control and Prevention data. Photo © J. Carl Ganter/Circle of Blue

Brett writes about agriculture, energy, infrastructure, and the politics and economics of water in the United States. He also writes the Federal Water Tap, Circle of Blue’s weekly digest of U.S. government water news. He is the winner of two Society of Environmental Journalists reporting awards, one of the top honors in American environmental journalism: first place for explanatory reporting for a series on septic system pollution in the United States(2016) and third place for beat reporting in a small market (2014). He received the Sierra Club's Distinguished Service Award in 2018. Brett lives in Seattle, where he hikes the mountains and bakes pies. Contact Brett Walton