As of Sunday (7/19/2026), the New York City Department of Health and Mental Hygiene announced that 74 cases of Legionnaires Disease had been identified in an Upper East Side cluster, resulting in three fatalities.
While contaminated cooling towers have been identified as the source, determining the source of an outbreak involves much more than finding Legionella in a water system.
Health investigators utilize the classic epidemiologic framework of time, place, and person to identify the most likely source of exposure.
- Time: Investigators determine when each patient became ill and reconstruct activities during the 2- to 14-day incubation period. Establishing a timeline helps identify when exposure likely occurred and whether cases are linked.
- Place: Investigators map where patients lived, worked, and traveled during the incubation period. Geographic overlap helps narrow the search to potential aerosol-generating water systems, such as cooling towers, domestic water systems, or decorative fountains.
- Person: Investigators evaluate who became ill, including age, medical conditions, occupation, and shared activities. Identifying common exposures among unrelated individuals is often the strongest evidence pointing to a common source.
From Epidemiology to Source Attribution
The epidemiologic investigation identifies where to look. Source attribution determines whether a specific environmental source caused the illnesses.
Environmental investigators evaluate building water systems, review water management practices, inspect system operation and maintenance, and collect targeted water samples for Legionella analysis. At the same time, healthcare providers obtain clinical specimens from patients using Legionella urinary antigen testing, supplemented by lower respiratory culture or molecular testing when possible. Culture-positive clinical isolates are particularly valuable because they can be compared with environmental isolates. This microbiological evidence, combined with epidemiologic findings and engineering evaluations, provides the strongest evidence for source attribution.
Prevention Starts Before an Outbreak
A comprehensive Water Management Program (WMP) remains the most effective way to reduce the risk of Legionella growth. Programs developed in accordance with ASHRAE Standard 188 and CDC guidance help facilities identify and control conditions that promote bacterial amplification before illnesses occur.
RHP Risk Management’s Comprehensive Water Management Plan Support
RHP Risk Management’s ASSE 12080 certified professionals develop and audit site-specific WMPs in accordance with ASHRAE Standard 188, CDC Toolkit guidance, and CMS requirements; conduct Legionella risk assessments, sampling, and outbreak investigations; and provide regulatory, legal, and operational support to help facilities of all sizes proactively manage risks, ensure compliance, and respond effectively to incidents.
RHP professionals bring extensive expertise to the investigation of buildings and suspected sources of Legionella, working closely with clients to develop tailored, site-specific methodologies and plans grounded in sound scientific and risk-based principles. We partner with organizations to address urgent concerns, clearly communicate identified risks, and outline practical, effective solutions to guide next steps with confidence.
Contact RHP Risk Management to schedule a consultation or request a review of your current plan. Proactive action today protects building occupants and operations tomorrow.