Why Legionella Cases Are Rising and What It Means for Your Building

Reported Legionnaires’ disease has climbed for two decades, and the newest state data confirms the trend is continuing upward. In 2025, New York recorded 6.1 cases per 100,000 people, up from the 4.1 rate that held steady from 2022 through 2024. New York is one state, but the conditions behind that Legionnaires’ increase sit in buildings everywhere: aging plumbing, oversized hot water systems, cooling towers, and an aging, more vulnerable population. It is a warning for every facility operator in the country.  

The numbers are moving in one direction

A nearly 50 percent single-year increase in a state the size of New York is not a statistical blip. It tracks with federal data: Legionella is the leading cause of waterborne disease outbreaks tied to building water systems in the United States. Better diagnostics explain part of the rise, but not all of it. The bacteria is finding more places to grow, and most are inside occupied buildings nobody is testing.

Healthcare facilities carry the heaviest burden

In 2025, New York investigated 107 healthcare-facility-associated clusters or outbreaks, against 33 in the community. That ratio is not an accident: hospital and skilled nursing populations are older, sicker, and immunocompromised, and large healthcare plumbing systems hold water long enough for Legionella to multiply. It shows up in the death rate. The case fatality proportion for healthcare-associated Legionnaires’ disease runs near 25 percent, more than double the 10 percent in community cases. A single linked case in a hospital is a sentinel event.  

Why a negative Legionella test does not clear your building

The most common Legionella test, urine antigen, only detects Legionella pneumophila serogroup 1. It cannot identify or rule out the other species and serogroups that cause disease, and New York’s advisory states this plainly. A patient can test negative and still have legionellosis from another species. The same blind spot exists on the environmental side: testing a building’s water for serogroup 1 alone misses other species fully capable of putting someone in the hospital.

Detection is not the same as finding the source

Confirming Legionella is present is the easy part. Finding what grew it protects people. New York’s guidance is direct: culture is the only method that can link a clinical case to an environmental source. A positive sample tells you that you have a problem, not which cooling tower, water heater set too low, or dead leg of pipe is feeding it. An engineering site investigation maps the system and finds the specific failure. Remediation that skips that step kills what is in the pipes today and leaves the conditions that regrow it next month.

What a facility should do

Start with an engineering investigation before any treatment decision. Map the water system, find the temperature zones, low-disinfectant areas, and stagnation points where Legionella grows, and test for all species rather than a single serogroup. Then build the remediation plan around what the investigation found. For occupied healthcare and correctional buildings, many remediation options work without evacuation, removing the highest hidden cost of a Legionella response. The rising case numbers are not a reason to panic. They are a reason to investigate before a case forces your hand.

An engineering site investigation runs $4,900 to $12,000 per building, with proactive risk assessments starting at $4,900. The price tracks building size, hot water loops and floors, water system complexity, and whether a health department is involved. Onsite field water quality testing for pH, free chlorine or chloramine, temperature, ORP, conductivity, hardness, and pressure is included in the per-building fee. Legionella culture testing is billed separately.

How to reduce your risk of a Legionnaires outbreak by 90%

The case that triggers a health department investigation is the expensive one. Almost everything that prevents it happens before anyone gets sick, at a fraction of the cost. Four steps, done in order and kept current, take most facilities from exposed to controlled.

Start with a risk assessment. You cannot control what you have not mapped. A risk assessment by a Legionella engineer documents every point where temperature, stagnation, or a low disinfectant residual lets the bacteria grow, from cooling towers to hot water loops below 120 degrees to dead legs and rarely used fixtures. It is the baseline every other step builds on.

Build a water management plan around those findings, or fix the one you already have. Most facilities under Joint Commission, CMS, or ASHRAE 188 expectations already have a plan on paper; the question is whether it reflects the actual building. A template plan names control points that do not exist in your system and misses the ones that do. The assessment ties it to your real equipment, with control limits that mean something.

Then confirm the plan is being followed. A plan that lives in a binder protects no one. Control measures have to be performed on schedule, by someone accountable, with readings logged: temperatures checked, residuals measured, flushing carried out where needed. Most facilities that get an outbreak had a plan. What they did not have was proof it was running.

Validate with consistent Legionella testing, at least once per quarter. The plan tells you what should be happening; testing tells you whether it worked. Quarterly culture testing for all Legionella species, not serogroup 1 alone, catches a control failure while it is still a number on a report, not a patient in a bed. Facilities that test once a year find out too late.

Do these steps consistently, and you remove the conditions behind the large majority of Legionnaires cases. That is what cutting your risk by 90 percent looks like in practice. Not a single product, but a program that is built right, followed, and checked.

To discuss a risk assessment or an engineering site investigation for your facility, call Legionella Control Systems at 888-416-8626.

Frequently Asked Questions

Why are Legionella cases rising across the country?

Reported legionellosis has climbed for two decades, and the newest state data confirms the trend has not slowed. New York recorded 6.1 cases per 100,000 people in 2025, up from 4.1 across 2022 through 2024. Aging water systems, a larger at-risk population, and improved testing all push the numbers higher.

What does the New York data mean for my building’s water system?

It means the bacteria already lives in building plumbing at a higher background rate than most operators assume. New York investigated 107 healthcare-facility-associated clusters or outbreaks in 2025, and Legionella does not respect state lines. The same cooling towers, hot water systems, and stagnant pipe runs sit everywhere.

Why are healthcare facilities at higher risk?

Two factors stack. Patients are older and sicker, which raises infection risk, and large healthcare plumbing systems hold water long enough for Legionella to multiply. The case fatality proportion for healthcare-associated Legionnaires’ disease runs near 25 percent, more than double the 10 percent in community cases.

Why does urine antigen testing miss most Legionella species?

Urine antigen testing only detects Legionella pneumophila serogroup 1. It cannot identify or rule out the other species and serogroups that also cause disease. A negative urine test does not clear a patient, and serogroup-1-only testing does not clear a building.

Why does finding the case clinically not fix the building problem?

A clinical diagnosis tells you a person is infected, not where the water system failed. New York’s guidance states that culture is the only method that can link a clinical case to an environmental source. Without an engineering investigation, the source stays in place and infects again.

What is the difference between detecting Legionella and finding its source?

Detection confirms the bacteria is present. Source identification traces it to the specific equipment, temperature zone, or stagnation point that grew it. Counting colonies tells you that you have a problem; finding the source tells you what to fix. Remediation that skips the second step removes the symptom and leaves the cause.

What should a facility do after a confirmed case is linked to its water system?

Start with an engineering site investigation before any treatment. Map the water system, identify where temperature, low chlorine residual, and stagnation favor growth, and test for all Legionella species. Then remediate based on findings. Occupied buildings, including healthcare, long-term care, and correctional facilities, can be treated without evacuation. Call 888-416-8626.