In a recent whitepaper written by Dr. Janet Stout and Abraham Cullom of the Special Pathogens lab – both legionella experts – it’s reported that legionnaires' disease is on the rise, particularly in the healthcare setting. Climate change, aging infrastructure and increasing population are all identified factors that increase legionella cases. Cullom points out that although there is a common perception that Legionella cases are on the rise, things have started to level off in the 2020s.

“Between the mid-90s and around 2018, reported cases increased exponentially. Part of that was likely due to better awareness and diagnosis, but the upward trend itself was still deeply concerning,” he says. “We saw a national standard in 2015—ASHRAE 188—which laid the foundation for water management programs in buildings. But it was voluntary at first, and it’s taken time for those practices to take hold across industries. Even now, adoption isn’t universal.”

Cullom points out that there are plenty of challenges ahead. “Climate change is impacting water sources and treatment practices and infrastructure is aging constantly. We’re better than ever at keeping people alive through cancer, transplant, and chronic illness—but that means more people are at risk from opportunistic pathogens like Legionella.”

To get a pulse for the state of legionella prevention in 2025, we sat down for a Q&A with Cullom to cover everything from COVID-19 to AI and smart plumbing devices.

How did the COVID-19 shutdowns put emphasis on Legionella outbreaks and prevention?

AC: Interestingly, reported Legionella cases and outbreaks actually went down during this period. You had fewer people traveling, going to hospitals for routine procedures, or occupying high-risk buildings. So, exposure opportunities dropped. But while the case numbers dipped, awareness of building water stagnation surged. Shutdowns created this perfect storm for stagnation—with warm, low-flow conditions and decaying disinfectant residuals. And suddenly, facilities that had never thought much about flushing or water management needed to.

The American Water Works Association (AWWA) put out key guidance during this time: Responding to Water Stagnation in Buildings with Reduced or No Water Use. That, along with CDC guidance and ASHRAE 188, helped shift the conversation from reactive testing to proactive prevention.

COVID-19 shutdowns didn’t create Legionella problems per se, but it reinforced that good water quality isn’t passive.

Your report mentions costs associated with the legionnaires' disease. Explain those costs and the best ways to mitigate them.

AC: These are the otherwise avoidable costs of treating these cases that burden our society. A CDC* analysis looked at the broader burden of waterborne diseases in the U.S., including Legionnaires’, and found that it’s among the costliest. They calculated direct healthcare costs—things like emergency room visits, hospital stays, and out-of-pocket payments. Legionnaires’ disease costs hundreds of millions of dollars each year here. And that doesn’t even touch indirect costs like lawsuits, building remediation, lost productivity, or the long-term health impacts on survivors.

The best cure here prevention. That means putting well-considered water management plan in place and testing for Legionella in water systems more broadly than we currently do. Clinical testing is just as important. Too often, cases go undiagnosed, or are delayed in diagnosis, because physicians aren’t thinking Legionella when someone shows up with pneumonia.

* Estimate of Burden and Direct Healthcare Cost of Infectious Waterborne Disease in the United States - Volume 27, Number 1—January 2021 - Emerging Infectious Diseases journal - CD

What new mandatory (regulatory) requirements should plumbing professionals be aware of?

AC: The biggest recent development is New Jersey’s Legionella law—S2188/A1970—which passed in 2024. It’s one of the most comprehensive Legionella regulations in the U.S., and it’s worth paying attention to—not just for folks in New Jersey, but also as a signal of where things may be headed in other states.

This is an interesting law in that it applies to both public water systems and building owners and operators. While other states have targeted cooling towers (e.g., New York) or specific facility types like schools or healthcare (as seen in Illinois and Pennsylvania), New Jersey’s law casts a wider net.

For many public water systems, the law sets minimum disinfectant residuals (0.3 mg/L for free chlorine, 1.0 mg/L for monochloramine) and requires a distribution system maintenance plan, as well as communication procedures.

On the building side, facilities like hospitals, nursing homes, senior housing, high-rises, and hotels must implement a water management plan that complies with ASHRAE 188 within 24 months, September next year. The law also gives the New Jersey Department of Health authority to mandate Legionella testing during outbreak investigations and requires facilities to report and publicly disclose the presence of Legionella if detected.

For anyone operating in or around New Jersey, I think being familiar with ASHRAE 188 and how to support a water management plan is really important now.

What non-mandatory precautions do you advise businesses take?

AC: A great starting point is the CDC’s Legionella Water Management Toolkit. It’s designed to help building operators and facility teams think through system design, operations, and maintenance with prevention in mind. From there, compare your building’s features against the Building Survey in ASHRAE 188. If your system has risk factors—like cooling towers, central hot water, or areas that serve vulnerable populations—it’s worth going the next step and developing a bonafide Water Safety & Management Plan.

But the big one is: test. The best-designed plans are ultimately just plans. Building systems are complex, there’s billions of bacteria in them, and very few of us. You need something to tell you if your beautiful plan is actually working like you thought it would. You wouldn’t jump out of a plane with a parachute that was “well-designed” but never tested. The same logic applies to your water system.

What challenges are we facing when it comes to preventing Legionella today?

AC: We’ve made real progress, mostly in healthcare, but want to keep advancing the ball. The huge majority of Legionella cases are sporadic, not tied to a known outbreak. That means there are risks flying under the radar in places without formal water management programs. One of the most important shifts we need is broadening the scope of where water management is applied. It’s not just hospitals and nursing homes—schools, high-rises, hotels, and even some multi-family housing can present real risk, especially for vulnerable populations.

Broadly, the mindset needs to change: Legionella mitigation isn’t a niche concern. It’s part of responsible building operations.

Are there misconceptions about Legionella or the disease that you can clear up?

AC: Probably the oldest one is that it’s just a “cooling tower disease.” While cooling towers have been linked to high-profile outbreaks, most Legionnaires’ disease cases come from domestic water systems—showers, sinks, and plumbing in everyday buildings.

And while I’d always prefer to see a building over-reacting than under-reaching, some people tend to think that any Legionella presence is a cataclysmic threat. The reality is that it’s a normal part of our building water microbiomes, present in probably about 50% of large buildings. So, it’s something most of us live with regularly. Keeping an eye on it, keeping it under control is the key to disease prevention, not necessarily chasing every positive everywhere down to zero.

Another misconception is that Legionella is one bug. It’s actually a whole genus of bacteria with 60+ species, but we often treat them as a monolith. L. pneumophila is the main culprit behind most cases, but other species can cause illness too—and may behave differently in water systems. These species belong in the same genus, but so do a grey wolf and my mom’s pug, Princess. We tend to assume that if a system is favorable for L. pneumophila, it’s the same for other species. But the reality is murkier. We have some indications that other species may prefer different environmental conditions. We don’t fully understand those niches yet, because they haven’t been studied nearly as much. In the absence of data, myths tend to fill the gap.

How do IoT devices, AI, and smart plumbing integrate/impact legionella prevention?

AC: So there’s some real promise here—especially with automated flushing systems and water quality monitoring.

Smart flushing systems can help maintain flow in underused parts of the building, which is crucial for preventing stagnation. And because they can be programmed based on usage patterns, they strike a balance between water safety and conservation—flushing just enough, but not more than needed.

On the monitoring side, we already know quite a bit about how water quality parameters—temperature, residual disinfectant, flow—relate to Legionella risk. Integrated monitoring systems can give facilities a heads up when these parameters are out of target ranges before they show up in a culture result.

It’s not a silver bullet, but these are great technologies for supplementing water management, and there’s a good trajectory in this space.

How does the nationwide emphasis on sustainability & water conservation impact legionella prevention?

AC: There’s a real tension here. Efforts to reduce water and energy use—obviously critical for sustainability—can sometimes work against Legionella prevention. For example, lowering hot water temperatures to save energy may inadvertently create conditions ideal for Legionella growth. Similarly, low-flow fixtures, extended pipe runs, and water reuse systems can all contribute to stagnation and increased water age if not carefully managed.

That said, modern tools like the IAPMO Water Demand Calculator are actually helping on that front. Unlike older methods like Hunter’s Curve, which tended to oversize pipes based on outdated usage assumptions, IAPMO’s approach reflects how people actually use water today—especially with low-flow fixtures. The result is smaller pipe sizing, which can mean lower water age and better turnover, both of which help reduce risk.

Smart flushing devices can maintain flow in rarely used fixtures and help prevent stagnation without wasting water. They let you to schedule or automate flushing based on real usage patterns, making it possible to stay proactive without compromising conservation goals.

What new actions or precautions do you see manufacturers, building designers/owners and contractors taking today to prevent legionella growth?

AC: We're seeing a broader awareness of Legionella risk across the industry—not just in healthcare anymore. Designers, manufacturers, and contractors are starting to tap into a well-developed body of knowledge that’s grown rapidly over the past decade.

On the product side, one key development is the introduction of Legionella-specific standards, like ASSE LEC 2011-2022.

On the design side, we’re seeing smarter pipe sizing and layout strategies, often guided by tools like the IAPMO Water Demand Calculator. Right-sized systems reduce stagnation, improve disinfectant retention, and help prevent dead legs—all of which lower risk.

That said, it’s still an uphill battle. The progress is real, but it’s uneven—and we need to keep our foot on the gas. Legionella prevention isn’t something we solve once and walk away from. It’s a continuous process of applying what we know, adapting to building-specific conditions, and staying engaged to end Legionnaires’ disease.