What Is Legionella -and Why Does It Keep Killing People?

Cases of Legionnaires' disease have risen by approximately 900 percent in the United States since the year 2000. That number is not a misprint, and it is not explained by better testing alone.

Legionnaires' disease carries a hospitalization rate of approximately 95 percent, and the national case-fatality rate is estimated at 10 percent for community-acquired cases and 25 percent for healthcare-acquired cases. For immunocompromised patients, the population most likely to be in the facilities where Legionella thrives - the mortality rate can reach 40 percent in severe cases.

Yet most of the buildings where exposure occurs show no visible signs of contamination. The water looks clean. It runs hot. Nobody flagged a problem. And then someone gets sick.

What Legionella Actually Is

Legionella pneumophila is a gram-negative bacterium that exists naturally in freshwater lakes and streams at concentrations too low to cause illness. The problem begins when it enters a building's water system and finds the conditions it needs to multiply.

Those conditions are not exotic. Warm water, ideally between 68 and 113 degrees Fahrenheit. Stagnation. Biofilm: the microscopic film that lines every pipe, fitting, and water storage vessel. Scale and sediment, which provide shelter and nutrients. These conditions exist in some form in virtually every large building with a complex water distribution system: hospitals, hotels, apartment complexes, nursing homes, office towers.

Legionella spreads through water systems in large buildings via showers, faucets, cooling towers, hot tubs, and decorative fountains. Infection happens not by drinking contaminated water but by inhaling it, fine aerosols produced wherever water is dispersed into the air. Healthcare-associated legionellosis accounts for approximately 20 percent of cases in the USA.

Why Cases Keep Rising

Reported cases of Legionnaires' disease have been increasing since the early 2000s, with a peak in 2018. After a dip during the COVID-19 pandemic, cases rebounded sharply in 2021 and have continued to rise.

Several factors converge to explain the trend. Aging building infrastructure means older pipes, more dead-leg plumbing, and more opportunities for stagnation. The COVID-19 pandemic left millions of square feet of commercial and healthcare building water sitting unused for months, ideal conditions for Legionella colonization. Underdiagnosis remains a significant concern: the actual incidence of Legionnaires' disease is estimated to be 1.8 to 2.7 times higher than reported figures.

The year 2024 saw a documented global surge. Multiple regions reported major outbreaks simultaneously: 114 cases and two fatalities linked to a cooling tower in Melbourne, Australia; 53 cases and four fatalities in Lombardy, Italy; 30 confirmed cases and two deaths in London; and multiple clusters across the United States, including a senior living facility in Albany, New York where four people died.

Who Is Most at Risk

Legionnaires' disease is especially severe in older people, smokers, and those with compromised immune systems. This is precisely why healthcare facilities, nursing homes, and senior living communities carry the highest risk profile, not just because their water systems are complex, but because their populations have the least physiological resilience to the disease.

The geography of risk also matters within a building. Showerheads and faucets in patient rooms are the highest-exposure points. So are ice machines and drinking fountains that see intermittent use. Cooling towers, common in larger hospitals and hotels, have been implicated in some of the largest documented outbreak clusters in history. Anywhere water is warmed, stored, or aerosolized carries potential exposure risk if the system is not actively managed.

The Gap Between Knowing and Doing

Regulatory bodies in the United States have understood the Legionella risk well enough to mandate action. The CDC's Water Management Program toolkit provides explicit guidance. CMS directive QSO-17-30 has required healthcare facilities receiving Medicare and Medicaid funding to maintain Water Management Programs since 2017. ASHRAE Standard 188 establishes the engineering framework.

Yet a 2022 survey found that only 42 percent of healthcare facilities had fully implemented ASHRAE 188-compliant programs, leaving the majority at risk during CMS surveys.

The gap is not usually ignorance. It is the complexity of implementation. A Water Management Program requires a multidisciplinary team, documented flow diagrams of every water system, identified control points, scheduled monitoring, and corrective action procedures. For a large hospital with hundreds of outlets, that is a substantial operational undertaking, and the paperwork alone does not make anyone safer.

The Question Worth Asking

The trajectory of Legionella incidence rising for two decades, with a documented global surge in 2024, raises a straightforward question for anyone responsible for a building's water systems: is the current program genuinely reducing the risk at the point of exposure, or is it primarily a documentation exercise?

System-level treatment, temperature management, chemical disinfection, periodic flushing is the foundation of any Water Management Program. But biofilm persists in pipe walls regardless of system-level chemistry. Dead legs accumulate stagnant water regardless of building-wide temperature protocols. And the moment a vulnerable resident or patient turns on a shower, they are exposed to whatever the pipe directly above the fitting has been growing.

Point-of-use filtration addresses precisely this gap, not as a replacement for a Water Management Program, but as the final physical barrier between a building's water system and the person using it. The engineering question is not whether to have a WMP. It is whether the WMP's last line of defense is adequate for the population it serves.

Questions about point-of-use filter specifications for your facility?    Contact the MWT team.