Legionella is a naturally occurring waterborne bacterium that becomes dangerous when it colonizes and amplifies in human-made water systems. The most common sources are cooling towers, but may also include hot tubs, decorative fountains, hot water tanks, and complex plumbing systems.
People are infected by inhaling contaminated water droplets (mist), not through person-to-person contact. The resulting illness, Legionnaires’ disease, is a severe pneumonia with an average fatality rate of roughly 10% – rising substantially in healthcare and long-term-care settings. Risk factors for contracting illness include age (over 50 years old), smoking, and individuals with a history of lung disorders or compromised immune systems. The onset of symptoms may occur 2-10 days after exposure. Legionella exposure claims frequently implicate property owners, facility management companies, and water-treatment contractors. Increasingly, cases of illness draw plaintiff firms’ marketing directly to potential claimants within days of a public health announcement.
This issue of HETI Horizons reviews the major confirmed U.S. Legionella events during the past 12 months, the causation patterns responsible parties should watch for, and the regulatory landscape shaping liability exposure. Several of these events, particularly the ongoing New York City cluster, were still under active investigation at the time of writing and figures may have changed since.
Major U.S. Outbreaks: July 2025-July 2026
Over the past year, there have been several Legionella events that resulted in illness and death. As of July 2026, in the Upper East Side in Manhattan, NY, CNN reported 74 confirmed cases of Legionnaires’ disease – including 12 people hospitalized and three fatalities with the illness. New York City (NYC) identified this as a likely community cluster with cases limited to three ZIP codes and by mid-July had inspected all 183 cooling towers in the Carnegie Hill/Yorkville investigation zone – finding 76 with preliminary Polymerase Chain Reaction (PCR) positive results. No definitive source building has been confirmed and officials noted that confirmatory culture testing was being conducted. This incident was the first major test of NYC’s tightened cooling tower inspection law, effective May 2026.
In Westchester County, NY, 37 cases of Legionnaires’ disease, including two deaths, occurred in the Summer of 2025. This incident included a five-case Yonkers sub-cluster in which investigators were unable to match patient sample results to any specific cooling tower.
A similar outbreak of Legionnaires’ disease occurred in July through August 2025 in central Harlem, NY. In this event there were 118 confirmed cases and seven deaths. Genomic sequencing matched clinical sample results to two cooling towers on the same city block – one of the more definitive source confirmations among last year’s clusters.
From August through September 2025, 74 cases of Legionnaires’ disease, including two deaths, occurred in Marshalltown, Iowa. State epidemiologists attributed the outbreak to a cooling tower. The business responsible was never publicly identified, shielded by Iowa’s communicable-disease privacy statute. Notably, Iowa has no statewide cooling tower registration or testing requirement and any future regulation would have to come from local ordinance.
In Ector County (Odessa), Texas,12 cases of Legionnaires’ disease, including two deaths, were reported from November 2025 through January 2026. Local health officials were unable to identify a single source, stating there was no reason to suspect municipal water and instead pointing to individual risk factors such as unsanitized home respiratory equipment.
Over the past year, there were a number of smaller clusters – including a gym in Orlando, Florida (14 cases, 0 deaths), a nursing facility in Bloomingdale, Illinois (2 cases, 0 deaths), a hospital in Cincinnati, Ohio (2 cases, 0 deaths), and a resort in Avon, Colorado (3 cases, 0 deaths).
Why the Trend Line Keeps Climbing
Centers for Disease Control and Prevention (CDC) surveillance data show reported Legionnaires’ disease cases have been rising since the early 2000s, peaked in 2018, and rebounded after a COVID-era dip. 2024 and 2025 provisional figures continue that upward trend, per CDC. Aging cooling tower and plumbing infrastructure; inconsistent water management program adoption; warmer and more humid summers that extend the seasonal risk window; and an aging, more comorbid population all contribute to sustained frequency. Detection is also improving in well-resourced jurisdictions, which increases reported case counts even where underlying exposure hasn’t changed – an important consideration when comparing loss trends across the United States.
Regulatory and Compliance Landscape
Regulations to control Legionnaires’ disease occurrences vary among the states. New York requires registration, annual certification, monthly inspection, and Legionella testing at least every 90 days for all cooling towers – with the law tightened further in May 2026 after the Harlem outbreak. Other states, including Iowa and Texas noted in the events above, have no comparable statewide mandate, leaving water management program (WMP) adoption largely voluntary and governed by industry guidance, such as ASHRAE 188, rather than statute.
Risk Management Takeaways
Reducing Legionella exposure is best achieved through a rigorous building-specific WMP and active case management. Some of the preventative measures that can be taken include:
- Properties with cooling towers, hot tubs, decorative fountains, or complex plumbing (hotels, hospitals, senior living, large multifamily, gyms, resorts) should have a documented, currently maintained WMP, and treatment, if needed.
- Frequency of testing should be in accordance with applicable state/local requirements. Voluntary compliance with ASHRAE 188 should be a minimum threshold – even where no statute exists.
- Source-attribution failure is a real possibility in claims handling; not every outbreak resolves to a single confirmed premises source, which affects both coverage triggers and subrogation potential.
- Rapid plaintiff’s bar activity should be expected following any public health advisory; claims development can begin before final case counts or environmental testing are complete.
How HETI Can Help
HETI’s team of Certified Industrial Hygienists and EHS professionals work with property owners, facility managers, and insurance carriers to reduce Legionella exposure before it becomes a claim. Our services include on-site water system risk assessments and cooling tower inspections; development and audit of water management programs aligned with ASHRAE 188 and applicable state/local requirements; coordination of Legionella sampling, culture, and both PCR or culture testing with accredited laboratories; post-remediation verification/documentation; and rapid-response consultation when a cluster or positive test result is identified.
By combining technical expertise with regulatory knowledge across jurisdictions, HETI helps clients close the gap between voluntary guidance and enforceable compliance – reducing both the likelihood of an outbreak and the liability exposure that follows one.
To find out more about HETI’s industrial hygiene and EHS services, please contact us.
Michael Henderson, PhD, CIH Director, Industrial Hygiene