Every so often a disease that sounds like it belongs to the 1970s lands back on the front page. Legionnaires’ disease is one of those. In the summer of 2025, a cluster in Central Harlem grew into one of New York City’s larger outbreaks in recent memory — 118 confirmed cases and seven deaths, ultimately traced to cooling towers on a single city block [1]. If you followed the news, you probably came away with two questions: what is this illness, exactly, and why does it keep happening? Both are worth answering clearly, because the honest reply is that Legionnaires’ is serious, it’s rising, and much of it is preventable.
Why this keeps coming up now
Legionnaires’ disease is a type of pneumonia caused by Legionella bacteria [2]. It got its name from a 1976 outbreak at an American Legion convention in Philadelphia, and it has never really gone away. What has changed is the trajectory: reported U.S. cases have climbed steeply since 2000 [3]. Why the increase is genuinely debated — likely some combination of aging water infrastructure, an older and more chronically ill population, warmer conditions favoring bacterial growth, and simply better testing and surveillance. I’d be cautious about attributing the rise to any single cause; the data support a multifactorial picture rather than one villain.
The practical point for readers is that these outbreaks are not freak events. They follow a recognizable pattern, and that pattern is what makes prevention possible.
What Legionnaires’ disease actually is
Legionella lives naturally in freshwater. It becomes a human problem when it colonizes building water systems — cooling towers, hot water tanks, showerheads, decorative fountains, hot tubs — where warm, stagnant water lets it multiply [2]. People get sick by breathing in contaminated mist or aerosol, not by drinking water and not, in general, from other people [2]. That last point matters: unlike the flu, you can’t catch Legionnaires’ from a sick coworker. The Harlem investigation is a clean illustration of the mechanism — health officials matched the bacterial strain in patients to the strain in specific cooling towers using genetic sequencing, and one of those towers hadn’t even been registered [1].
Symptoms look like other pneumonias: cough, fever, shortness of breath, muscle aches, sometimes confusion or diarrhea. There’s nothing about the bedside picture that screams “Legionella,” which is exactly why it gets missed. And this isn’t a mild illness. About 1 in 10 people who develop Legionnaires’ disease die from it; for infections acquired in a healthcare facility, where patients are already sick, it’s closer to 1 in 4 [2].
Who’s most at risk
Most healthy people exposed to Legionella never get sick. The illness concentrates in a fairly predictable group: adults 50 and older, current or former smokers, people with chronic lung disease, and those who are immunocompromised or living with conditions like diabetes, kidney or liver disease, or cancer [2]. In the Harlem cluster, the large majority of those sickened had at least one of these risk factors [1]. If you or a family member fall into these groups, that’s not cause for alarm, but it is a reason to take respiratory symptoms seriously and mention any recent travel or known exposure to a clinician.
Why it’s easy to miss — and why that’s fixable
Here’s the part I find most clinically important, because it directly affects outcomes. The rapid test most hospitals reach for is the Legionella urinary antigen test. It’s fast and convenient — but it only detects Legionella pneumophila serogroup 1 [6]. That serogroup causes most cases, so the test is genuinely useful, but a meaningful minority of infections are caused by other serogroups or species that a urine test will simply miss. For that reason, the CDC recommends pairing the urinary antigen test with a culture (or molecular test) of a lower respiratory specimen [6]. Culture does two things a urine test can’t: it catches the cases the antigen test misses, and it lets public health labs match a patient’s strain to an environmental source — which is precisely how outbreaks get solved [1,6].
Professional guidelines don’t call for testing every pneumonia patient for Legionella. The ATS/IDSA community-acquired pneumonia guideline reserves Legionella testing for severe pneumonia or when there’s an epidemiologic reason to suspect it — an outbreak, a cluster, or relevant travel [5]. That travel point is easy to underestimate. A recent CDC report described Legionnaires’ cases in travelers who stayed at two U.S. Virgin Islands hotels, where the bacteria were found not only in hot water but in cold-water systems kept warm by a tropical climate [4]. A returning traveler with pneumonia deserves a travel history, every time.
Treatment: the right antibiotic, early
Legionnaires’ is treatable, and early appropriate treatment clearly improves the odds. The catch is that Legionella lives inside human cells, so it doesn’t respond to some of the antibiotics used routinely for pneumonia. Effective therapy means a drug that penetrates cells — a respiratory fluoroquinolone such as levofloxacin or moxifloxacin, or a macrolide, usually azithromycin [5]. This is why considering the diagnosis matters as much as confirming it: for a patient sick enough to be hospitalized, empiric coverage that includes an anti-Legionella agent shouldn’t wait for a test to come back.
I’d add one honest caveat: the head-to-head evidence comparing fluoroquinolones and macrolides, and the ideal treatment duration, comes largely from observational studies rather than large randomized trials [5]. The first-line choices are well established; the fine-tuning is less certain than patients might assume.
The real story is prevention
If there’s a single takeaway, it’s that Legionnaires’ disease is mostly a building-water problem, which means it’s largely an engineering problem — and that’s good news, because engineering problems can be managed. The core tool is a water management program: a written plan that maps a building’s water systems, identifies where Legionella could grow, sets control measures (temperature, disinfectant levels, flushing stagnant lines, cooling-tower maintenance), and monitors that they’re actually working [7].
This isn’t just best-practice advice. The consensus standard, ASHRAE 188, lays out the framework, and since June 2017 the Centers for Medicare & Medicaid Services has required hospitals and long-term care facilities to maintain such programs to keep their Medicare and Medicaid certification [7,8]. Legionella grows best in warm, stagnant water roughly between 77°F and 113°F, so much of prevention comes down to keeping hot water hot, cold water cold, and water moving [7].
Cooling towers deserve special mention because they can aerosolize bacteria and spread them across a neighborhood — which is what makes them a repeat source of large community outbreaks. After a 2015 outbreak, New York City adopted some of the country’s strictest cooling-tower rules, requiring registration and frequent monitoring [1]. And yet Harlem still happened in 2025, with one implicated tower unregistered [1]. The lesson isn’t that regulation failed; it’s that prevention is only as good as its execution and enforcement.
There’s also a fairness dimension worth naming. Outbreaks tend to hit older, medically vulnerable, and sometimes under-resourced communities hardest, and questions about why certain neighborhoods repeatedly bear this risk are legitimate. I don’t think we should paper over them.
What we still don’t know
A few genuine uncertainties remain. We don’t fully understand why national case counts keep rising, and how much of that trend reflects a true increase versus better detection [3]. Because the common urine test misses non-serogroup-1 infections, the real burden is probably underestimated [6]. And the surveillance system itself has gaps — the CDC’s Virgin Islands report flagged delays in identifying cases and underreporting [4]. These aren’t reasons for alarm, but they are reasons for humility about the numbers.
Bottom line
Legionnaires’ disease is a serious pneumonia that is frequently preventable and, when caught early, usually treatable. For most people, everyday risk is low. The people who should be most attentive are those over 50, smokers, and anyone with chronic lung disease or a weakened immune system — for them, unexplained pneumonia, especially after travel or during a known cluster, is worth flagging to a clinician promptly, and worth asking whether Legionella has been considered. The larger fix, though, lives in buildings, not clinics: well-run water management programs and properly maintained cooling towers prevent far more cases than any antibiotic ever will.
What questions do you still have about Legionnaires’ disease or how outbreaks get investigated? Share them in the comments — but please keep it general and don’t post personal health details or ask for individual medical advice in a public thread.
This article is for general educational purposes and does not replace individualized medical care. If you’re concerned about symptoms or your personal risk, talk with a qualified healthcare professional.
References
- New York City Department of Health and Mental Hygiene. Outbreak of Legionnaires’ Disease Associated with Cooling Tower Systems in Central Harlem. NEJM Evidence (Public Health Alerts). 2026. doi:10.1056/EVIDpha2600062.
- Centers for Disease Control and Prevention. About Legionnaires’ Disease.
- Centers for Disease Control and Prevention. Legionellosis Surveillance and Trends.
- Centers for Disease Control and Prevention. Legionellosis Outbreaks Associated with Two Hotels — U.S. Virgin Islands, October 2024–April 2025. MMWR. 2026;75(25).
- Metlay JP, Waterer GW, Long AC, et al. Diagnosis and Treatment of Adults with Community-acquired Pneumonia: An Official ATS/IDSA Clinical Practice Guideline. Am J Respir Crit Care Med. 2019;200(7):e45–e67. doi:10.1164/rccm.201908-1581ST.
- Centers for Disease Control and Prevention. Laboratory Testing for Legionella.
- Centers for Disease Control and Prevention. Developing a Water Management Program to Reduce Legionella (Toolkit); aligned with ANSI/ASHRAE Standard 188-2021.
- Centers for Medicare & Medicaid Services. Requirement to Reduce Legionella Risk in Healthcare Facility Water Systems. Survey & Certification memo S&C 17-30 (June 2017; rev. 2018).
