By Sarahann Rackl, Bryson Brewer, and Mark Fecke

Introduction 

Legionnaires’ disease and the detection of Legionella, the bacteria that causes Legionnaires’ disease, are once again making headlines. In New York City, more than 50 cases of Legionnaires’ disease — and more than 20 hospitalizations — have been reported as of July of this year.¹ New York City officials have reported that 31 cooling towers tested positive for Legionella out of more than 180 tested,² with positive detections including a Metropolitan Transportation Authority subway station and the Guggenheim Museum. In Chicago, a federal judge believes he contracted Legionnaires’ disease from the Dirksen Federal Courthouse, where signs were posted at some drinking water fixtures stating that Legionella testing was outside of the “corresponding threshold.”³

These recent reports point to a broader, recurring risk for building owners and operators, as Legionella can proliferate in complex building water and HVAC systems. In 2025, 114 individuals became sick, and seven people died over the summer, in an East Harlem, New York outbreak that was also attributed to cooling towers.⁴ Chicago also made headlines in 2025 when Legionella was detected in the Environmental Protection Agency’s office buildings⁵ and a nursing facility was reported as a site of a Legionnaires’ outbreak.⁶ 

New York and Chicago are not unique in their Legionnaires’ disease outbreaks. Both the US and EU Centers for Disease Control (CDC) have reported a continued increase in the number of cases and outbreaks since 2000, with the highest incidents occurring in the summer months.^7,8 

How is Legionnaires’ Disease Acquired? 

Legionnaires’ disease is typically acquired through the inhalation of water droplets that contain Legionella bacteria. Legionella can grow and proliferate in engineered water systems like drinking water systems and Heating, Ventilation, and Air Conditioning (HVAC) systems. Water droplets containing Legionella can form from plumbing fixtures, such as shower heads and faucets, as well as from spas, fountains and other water features, and HVAC systems. 

Management of Building Drinking Water Systems and HVAC 

Understanding your building’s water and HVAC systems’ potential for Legionella proliferation and transmission is critical to developing effective control strategies, mitigating the overall transmission risk, and responding appropriately if and when detections occur.

In the United States and most other countries, building owners are responsible for maintaining drinking water quality within their plumbing and HVAC systems at the point the water enters the property from the public water system. Organizations including the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE), the US CDC, and the American Industrial Hygiene Association have published recommendations that can help building owners and operators assess vulnerabilities to Legionella growth, identify conditions that may allow the bacteria to proliferate, and develop water management plans and monitoring programs to mitigate the potential presence of Legionella and associated risks in the future. However, implementing effective water management plans, operations, and maintenance programs to mitigate and control Legionella proliferation is highly building- and system-specific, and older buildings often require a more nuanced approach.

Effective monitoring can confirm whether water management plans and maintenance programs are functioning as intended. However, interpreting and responding to the results of such monitoring can be difficult. While the presence of a few Legionellae does not necessarily mean there is an imminent risk of infection, there is a need to respond when management and operations to control Legionella are not preventing its proliferation. Conversely, overtreating systems and responding to every additional Legionella detection can be costly and result in potentially costly damage to plumbing and HVAC system materials.

How Secretariat Can Help 

Before incidents occur, or if you receive a positive Legionella detection, or if a reported Legionnaires’ disease case is associated with your building, Secretariat’s building drinking water and HVAC system engineers, microbiologists, and other experts can help you conduct: 

• Audits of your water systems, water system management plans, and HVAC system management operation programs for Legionella and other opportunistic pathogen control 
• Assessments of Legionella and water quality monitoring and results to evaluate the effectiveness of your building water management program 
• Root Cause analysis of your building HVAC and building potable water systems and features to help pinpoint the source of Legionella proliferation
• Design reviews of new or existing building HVAC and potable water systems and the Legionella control mechanisms 

1. Legionnaires’ disease outbreak Upper East Side: Testing reveals buildings possibly linked to cluster – ABC7 New York ↩︎
2. NYC Legionnaires’ outbreak: 31 cooling towers test positive for Legionella – ABC7 New York ↩︎
3. Durbin pressing for action amid possible Legionella at Dirksen Federal Courthouse in Chicago ↩︎
4. A seventh person has died in a Legionnaires’ outbreak traced to cooling towers in Harlem ↩︎
5. Legionella bacteria, lead found in EPA Chicago offices’ drinking water – Chicago Sun-Times ↩︎
6. Legionnaires’ outbreak linked to Chicago-area nursing facility, prompting health officials’ alert – CBS Chicago ↩︎
7. Legionellosis Surveillance and Trends | Legionella | CDC ↩︎
8. European Centre for Disease Prevention and Control. Legionnaires’ disease. In: ECDC. Annual epidemiological report for 2024. Stockholm: ECDC; 2026. Stockholm, May 2026  ↩︎