NYC is in the middle of a large Legionella outbreak, and the city is racing to contain it. Notably, this response feels different. For the first time, buildings that test PCR positive are being named publicly, more than 100 health department employees are working nonstop on the response, and communications have been far clearer than during previous outbreaks. This week, emergency physician and friend of YLE Jeremy Faust wrote a great piece highlighting the many ways New York is getting the response right. I’m excited to share his perspective in this guest post. And I’ll be back with some of my own thoughts later this week.

Lindsey Nicholson/UCG/Universal Images Group via Getty Images.

Even its name should summon effective public health: Legionnaires’ Disease. The designation was initially suggested by the media during an intense investigation into a frightful pneumonia cluster at a 1976 gathering of The American Legion. Eventually, though, after successfully pinning down the cause, CDC scientists embraced the moniker too.

In the famed inquiry, now considered a classic case study still taught in schools, CDC sleuths determined that the responsible pathogen was not primarily spreading from person-to-person. Instead, a common source was found—the convention hotel’s air conditioning system. Bacteria had grown in the cooling towers. Emanating mists eventually circulated throughout the hotel. Soon, over 180 became ill, and 34 died.

So, both the common and official scientific name, Legionella pneumophila, convey the end result of a process, the hard, essential work of public health. The very one that has been under attack during the second Trump administration.

Recall what then-presidential hopeful Robert F. Kennedy Jr. said in 2023, prior to becoming our nation’s top health official in 2025: “We’re going to give infectious disease a break for about eight years.”

That was profoundly stupid then. It’s even worse now.

Juxtapose that against a contrasting idea: “When public health works, there are no headlines.”—Unknown.

There, friends, are your extremes. In the first case, there is sympathy for the ostrich, head buried, tail up. In the second, inglorious (even mundane) behind-the-scenes preparation anticipates future threats by acknowledging that crystal balls don’t exist. The only way to prepare for one rare event is to prepare for many.

The only way to stop outbreaks are to discover them.

Clearly, New York City officials subscribe to the second approach. Actually, when it comes to modern approaches to Legionnaires’ Disease, they didn’t just read the book—they wrote important new chapters in it. Accordingly, they’ve done everything possible to keep a recent outbreak of Legionnaires’ Disease relatively beige.

While not as newsy as incompetence would be, it’s worth celebrating good technique. In fact, that’s what Inside Medicine is for. So, let’s talk about why public health is winning New York City’s current battle with Legionnaires’ Disease.¹

What’s going well.

Let’s check in on five domains in New York’s response. Data, resources, technique, innovation, and communication.

1. Data. The city is providing updates on case counts. Easy-to-find, easy-to-understand information reassures the public that their leaders are engaged, and that they trust their constituents. As of Sunday, there were 59 total cases (15 currently hospitalized, 33 discharged, 11 not hospitalized, and 0 deaths). Consider the opposite posture—Trump’s infamous line in the early days of Covid, “If we didn’t do any testing, we would have very few cases.” The ostrich approach insults the public’s intelligence, and increases risk. New York City has chosen the better path.
2. Resources. Once an outbreak has been detected—if you detect it at all—the ground game begins. In a Legionnaires’ outbreak, finding the source is everything. To do that, you have to test the water from the cooling tower systems in the area where cases are popping. To do that, you have to have people. (This is why you don’t mindlessly RIF public health employees, Elon.) Here, the city deployed its teams, over 100 staff, to the Upper East Side of Manhattan, to test tower-by-tower for Legionella. That’s how you do this.
3. Technique. New York City officials’ technical prowess centers on not waiting around. Rather than waiting for case counts to mount before deploying teams, city policy mandates an investigation whenever two or more potentially linked cases have been identified. In this case, water cooling towers are being tested, even if there have not yet been cases among residents served by them. On Friday, officials announced that water cooling towers serving 31 buildings had been identified. Rather than stoking panic, this information directed action. By Sunday, all of the relevant infrastructure had already been cleaned, meaning it was safe. And because every water cooling tower in the area was tested quickly, the response could be right-sized. There was no need to treat the many units that tested negative (and have not been associated with suspected cases). New York City officials spurned the ostrich.
4. Innovation. In prior outbreaks, health officials tested water cooling towers rapidly but waited until bacterial cultures were positive before requiring full cleaning and disinfecting. Here’s what the Mayor’s office wrote in an email about the new policy:

It’s important to note that more isn’t always better. There’s such a thing as overdoing it. But this escalation makes sense. Waiting for the confirmatory step (bacteria growing out in culture) would therefore represent an unnecessary delay. So, in this case, doing more sooner makes sense.

5. Communication. The city’s health department is way out in front of this. There’s no need for a university or media publication to throw up a dashboard because the city is providing official counts, with frequent updates. They’ve clearly messaged that there may be more cases, owing to incubation times, and this is to be expected. They’ve given their residents the information they need without fear-mongering. They’ve published press releases, updated their health information both for the public and for public health professionals/medical providers, and held public town halls. They’ve also cooperated with local and national media. Heck, they even answered my annoying questions all weekend.

Leading by example. “No notes.”

How often do we find ourselves bemoaning something broken in public health? Too often. So, it’s refreshing to see public health crushing it. Yes, I’m biased, because New York City’s health commissioner Dr. Alister Martin is a friend and former mentee. That also means that if he and his team were fumbling, I’d be working the phones trying to advocate for course corrections. In this case, I’ve got virtually nothing to offer them by way of advice, so far.

But don’t take my word for it. I asked Dr. Demetre Daskalakis, former Director of the CDC’s National Center for Influenza and Respiratory Diseases (who previously served in New York City’s health department from 2014-2020) what he thought of the current response. He agreed that the department has performed admirably here.

“No notes,” he added.

In a time when federal programs have been hampered by cuts, making it far harder to respond to new problems (#ExplosiveDiarrhea), New York City’s prompt actions in the current Legionnaires’ outbreak are a welcome reminder that public health, done by genuine experts, actually works.

Resources:

Some of New York City’s resources, including two peer-reviewed publications on their approach to Legionnaires’ Disease:

• Legionnaires’ Disease Cluster on the Upper East Side.
• Legionnaires’ Disease and Building Evaluations. (An interesting technical readout, for anyone interested.)
• Outbreak of Legionnaires’ Disease Associated with Cooling Tower Systems in Central Harlem. (This came out in late June, describing a 2025 outbreak. Nice timing.)
• Spatio-Temporal Cluster Detection for Legionellosis using Multiple Patient Addresses.

CDC information:

• About Legionnaires’ Disease.
• Controlling Legionella in Cooling Towers.

  ---

1. Transparency check: New York City’s Health Commissioner, Dr. Alister Martin, is a friend and former mentee of mine (Jeremy’s). I’ve agreed to participate in a “NYC Health Preparedness and Communications Roundtable,” but I have not yet done so, nor have I advised anyone in the department on this issue.

Well, nothing like the U.S.’s biggest explosive diarrhea outbreak ever to take me out of vacation mode.

But I’m getting really frustrated with the coverage, which leaves people struggling to navigate which foods are safe and which aren’t.

Some of this confusion is unavoidable, as outbreak investigations are messy and uncertain, unfolding quickly in real time.

But some of it is a direct result of what happens when the glue holding public health together (i.e., the federal government) is simultaneously gutted, lacking transparency and centralized communication, and impacted by corporate interests.

So here’s what we know, what we don’t, and what it means for you.

What we know

• The culprit is a microscopic parasite called Cyclospora, which spreads through food or water contaminated with human feces. This parasite lives in the human gut and is shed in stool as a tough, thick-walled shell. This is why it survives on produce and resists rinsing and typical sanitizers.
• This outbreak, the largest in U.S. history, has topped 3,000 cases, and it’s not slowing down.
•  That number comes from adding up state-level case counts, which are running well ahead of the counts on CDC’s website. For context, the U.S. typically sees around 3,000-4,000 cyclosporiasis cases in an entire year—so this single outbreak has already matched a normal year’s total, and it’s still climbing.
  • Note that the real number is almost certainly higher: most people with a few days of watery diarrhea ride it out at home rather than see a doctor, and those who do see a doctor may or may not get tested for cyclosporiasis (it’s an expensive test!).

This is now a little outdated, as it shows 1,500 cases in red, and the count has since doubled.

• This outbreak is nationwide, with more than 31 states reporting cases.
• Michigan has the most cases—more than 1,600 confirmed—but that doesn’t mean it started there. It is more likely to mean Michigan is better resourced for testing, reporting, and epidemiological legwork, so more of its cases are getting caught and counted. Michigan cases continue to increase quickly.

Figure from Force of Infection

• This outbreak is ballooning quickly because the U.S. hasn’t publicly pinpointed the contaminated food,
• NOT because it’s contagious person-to-person. When someone sheds the parasite in their stool, it isn’t immediately infectious—it needs 1 to 2 weeks in the environment to mature before it can make anyone else sick. Practically speaking, even without perfect hand hygiene, you’re not going to pass this to your family the way you would a typical stomach bug (called norovirus).
• Public health cuts have played a role here, but not the one getting the headlines.
•  A lot of blame is landing on last year’s cuts to FoodNet, a CDC surveillance program.
• But FoodNet was never designed for real-time outbreak detection or response. Instead, it tracks longer-term background trends for research. The more accurate culprits are the lack of centralized coordination by the federal government, our siloed health systems, and insufficient capacity at state and local health departments. The work is laborious, and public health is chronically underresourced. Local public health departments are doing this while tracking everything else, like measles. If the American public wants a public health system, then the U.S. needs to pay for it.
• One chain has already pulled an ingredient: Taco Bell. And the number of entertaining tweets about this (Taco Bell leading the appropriate response to diarrhea) is not in short supply.

What we don’t know (with some educated hunches)

• Why is it taking so long to find a source? 
• The only way to determine the cause is through epidemiological investigations (i.e., interviews with people to identify a common source) followed by lab testing. Interviews are particularly hard for cyclosporiasis because the person needs to recount food over the past 10 days. BUT, by now, hundreds of interviews have been completed, and there should already be signals from the noise. If there are signals about potential sources (which I’m hearing there may be), the information needs to be communicated (and quickly) at a national level. Regardless of whether there is a single definitive source or multiple potential ones, the longer the federal government takes to identify and name it, the more illness and hospitalizations we will see. It’s that simple.
• How did the contamination start? Still unclear. Broadly, there are two possibilities: contamination at the farm level (poor field sanitation, such as workers without adequate bathroom access, or contaminated irrigation water) or contamination during processing (typically via contaminated water at the packing or washing stage).

What this means for you

I know most of us would like to avoid having explosive diarrhea, but keep in mind that the risk is low (much lower than getting norovirus right now). Produce is healthy for you, and there are ways we can lower the risk without cutting it out completely.

This is how I’m thinking about it for my own family:

• Avoid the bagged or boxed salads for now, until there’s more clarity on which products and sources are affected. Whole heads of lettuce you wash and cut yourself are the safer bet in the meantime.
• Until a source is confirmed, lean toward produce with the least human handling. This means things you can peel, wash thoroughly, or cook. Produce with lots of bumps, grooves, and folds (leafy greens, berries) gives the parasite more places to hide, which makes it harder to wash off completely.
• Washing helps, but it won’t fully remove the parasite. Water removes some of it, but not all, because it clings and hides in nooks and crannies. Cooking will kill it, though I recognize “just cook your salad” isn’t a satisfying solution.
• If you are sick, be sure to talk to your clinical care team. Keep in mind:
  • Severe cases can be treated with antibiotics (called Bactrim). An older randomized controlled trial in Nepal showed that after treatment, parasites were detected in only 6% of patients (compared with 88% in the placebo group).
  • Symptoms can go away and come back for weeks. In one past outbreak, the longest case lasted 107 days. If your diarrhea keeps returning like this, it’s more likely cyclosporiasis than norovirus, so ask your doctor to test for it.
  • If a health department calls you for an investigation (i.e., interview), please participate. It will help others avoid the misery you’re experiencing and increase the likelihood of accountability down the road.
• Other questions I’m seeing:
  • Will you know if it’s on food because it smells like poop? No, these parasites are so small you won’t pick them up with smell.
  • Is this deadly? It hardly ever is. Most cases are mild, and severe cases can be treated. CDC is reporting 86 people hospitalized and no deaths.
  • If I’m old or pregnant, I’m already more worried, but is that warranted? Pregnant women, older adults, young kids, and anyone who is immunocompromised are more likely to get severely dehydrated or have a longer illness. Don’t tough out watery diarrhea. Call your clinical care team.
  • If I don’t go to the doctor, how is my data counted? It mostly isn’t. If you ride it out at home or your doctor doesn’t order that test, you’re not in the official count, which is a big reason officials think true case numbers are higher than reported.
  • Will a parasite cleanse work? No. The over-the-counter kits are typically some combination of herbal supplements and laxatives, and there’s no evidence they clear Cyclospora or any other parasite.

Bottom line

This is a very large foodborne illness outbreak, but there’s plenty we can do as individuals while we wait for government systems to catch up.

Now back to vacation. Be back next week with more!

Love, YLE

To get a deeper download, be sure to check out The Evidence Collective post yesterday.

A huge thanks to my friend and epidemiologist Dr. Caitlin Rivers over at Force of Infection for staying on top of the numbers and graphs so I didn’t have to while on vacation. Teamwork makes the dream work.

Your Local Epidemiologist (YLE) comprises a team of experts, ranging from physicians to immunologists to epidemiologists to nutritionists, working together with one goal: to “translate” ever-evolving public health science so that people are well-equipped to make evidence-based decisions. The YLE suite of newsletters reaches over 475,000 people across more than 132 countries. This newsletter is free to everyone, thanks to the generous support of fellow YLE community members. To support the effort, subscribe or upgrade below:

Subscribed