What Is Legionnaires’ Disease?
Legionnaires’ disease is a serious type of pneumonia (lung infection) caused by Legionella bacteria. While it primarily affects the lungs, if left untreated it can lead to dangerous complications, including organ failure. The appropriate antibiotic treatment is critical for recovery, which is why clinical awareness and early detection are so important.
In order for someone to contract Legionnaires’ disease, two things must happen:
- There must be a source of Legionella.
- There must be an exposure—the bacteria must reach the lungs, usually by inhaling contaminated aerosolized water droplets or by aspirating while drinking contaminated water.
The incubation period for Legionnaires’ disease to develop ranges from 2 to 14 days.Symptoms include fever, cough, muscle aches, and shortness of breath. Legionnaires’ disease is not spread person-to-person, it is a waterborne illness linked to contaminated water systems.
Sadly, about 10% of community-acquired cases are fatal, and the risk is even higher in healthcare-associated outbreaks. Older adults, smokers, people with chronic conditions like diabetes, and immunocompromised individuals face the greatest risks. In these vulnerable populations, mortality rates can climb as high as 30–50% if untreated.
The good news? Legionnaires’ disease is preventable. Understanding where and how Legionella bacteria grow is the first step toward controlling the risk.
What Is Legionella?
Legionella bacteria naturally occur in freshwater environments such as lakes, rivers, and streams. In nature, they rarely pose a problem as they exist in lower concentrations. The danger begins when these bacteria enter man-made water systems, such as cooling towers, plumbing networks, or hot water systems, where conditions can be favorable for them to multiply.
There are over 60 known species of Legionella, with about half linked to human disease. The most concerning species is Legionella pneumophila, specifically serogroup 1, which causes roughly 90% of Legionnaires’ cases.
Since Legionella cannot be identified by sight or smell, water testing is essential to detect its presence and guide the right corrective measures.
Common Sources of Legionella Growth
Any system or device that stores or circulates water can harbor Legionella bacteria if it’s not properly maintained. Key sources include:
- Cooling towers – Recirculated warm water and nutrients provide ideal growth conditions. If contaminated, drifting droplets can spread bacteria in aerosols for miles.
- Hot water heaters & storage tanks – If temperatures aren’t properly maintained (> 140℉), bacteria can thrive.
- Plumbing systems – Stagnant water in unused pipes or plumbing dead ends can create breeding grounds.
- Decorative fountains, water features, hot tubs, and spas –pose a risk for Legionella exposure because they can aerosolize warm, stagnant water that supports bacterial growth, allowing people to inhale contaminated droplets..
- Ice machines – Narrow tubing and biofilm buildup make them an overlooked but common risk.
- Medical devices – Respiratory equipment such as CPAP machines, oxygen condensers, or even humidifiers, and fan misters filled with contaminated tap water can spread bacteria via aerosols.
The takeaway: if water can stagnate or be aerosolized, it can potentially spread Legionella.
A Real-World Case: The Harlem Outbreak
In July 2025, an outbreak of Legionnaires’ disease in Central Harlem made national headlines.
- Initial cases were reported July 25.
- Within a week, the New York City Department of Health confirmed 22 cases and 1 death.
- The numbers quickly rose to 114 confirmed cases and 7 fatalities.
Investigations traced the outbreak to 12 contaminated cooling towers testing positive for Legionella pneumophila. Despite existing regulations, like New York City’s Local Law 77, which mandates inspections and water testing, this outbreak revealed that compliance alone is not enough.
The Harlem outbreak mirrors the 2015 Bronx outbreak (120 cases, 15 deaths) and underscores the urgent need for proactive water management beyond basic regulations.
Regulations and Standards
Preventing Legionnaires’ disease requires understanding the regulatory landscape. Some of the most recognized standards include:
- ASHRAE Standard 188 – Guideline for managing building water systems.
- CMS Requirements – healthcare facilities must develop and implement water management programs to prevent Legionnaires’ disease, in accordance with industry standards and guidelines such as ASHRAE 188.
- Local and state laws – NYC Local Law 77, New Jersey’s S2188, and others.
Since requirements vary by location and industry, consulting experts like IWC Innovations will ensure compliance and water safety across all facility types.
How to Prevent Legionella Growth
Prevention isn’t about a single action, it’s about a comprehensive water management strategy that covers all risk points. Core practices include:
- System Design & Maintenance – Avoid stagnation, plumbing dead legs, and oversized systems.
- Temperature Control – Keep hot water hot (≥140°F) and cold water cold (<77°F).
- Cleaning & Flushing – Prevent sediment accumulation, biofilm buildup, and flush infrequently used outlets regularly.
- Water Management Programs (WMPs) – Document risks, monitoring, and corrective actions.
- Proactive Testing – Detects issues early with CDC ELITE-certified lab testing.
- Corrective Measures – From point-of-use filters to chemical disinfection.
- Education & Training – Ensure staff and occupants know how to respond to risks.
By layering these strategies together, facilities can reduce the risk of Legionella growth. Prevention requires diligence, consistency, and a proactive mindset and small oversights can quickly turn into big problems.
Taking Action: From Detection to Education
Even the best-designed systems can encounter challenges, which is why preparedness matters:
- Routine testing allows facilities to detect issues early.
- Point-of-use filters provide short-term protection for vulnerable populations during troubleshooting, serving as a temporary measure until comprehensive water safety management is established.
- Disinfection treatments reset systems during outbreaks or case investigations.
- Staff training ensures that everyone—from maintenance to leadership—is ready to act quickly.
Ultimately, the goal is not zero detections of Legionella bacteria, but zero cases of Legionnaires’ disease.
The Bottom Line
Legionnaires’ disease is serious, but preventable. With proactive water management, regular testing, and team-wide education, facilities can protect their occupants and stay ahead of risks.
At IWC Innovations, we partner with facilities nationwide to create tailored water management programs, conduct Legionella testing, provide consulting expertise, and offer treatment services through all stages of water management. We also support and advocate for our clients in their interactions with Departments of Health across the country, helping ensure they meet regulatory expectations and protect building occupants through effective water management. Don’t wait for an outbreak to force action—take control of your water system today.
