In just 10 days over the summer of 1854, 500 people died of cholera in the Soho neighborhood of London. The city’s population had more than doubled to 2.3 million people in the first half of the 1800s, and its sewage system could not keep up. But the streams of human waste flowing into the street and seeping into the water supply were considered unconnected to the cholera crisis. The prevailing theory of the day was that bad air — miasma — caused illness.

The English physician John Snow thought differently. Five years before the outbreak he had suggested that the diarrheal disease was actually caused by a waterborne infection rather than miasma. He soon had a chance to test his theory, mapping the location of cholera-related deaths in Soho. Snow realized that the victims used one specific water pump on Broad Street, and he persuaded city officials to remove the pump’s handle to prevent anyone else from using it. With the source eliminated, the outbreak, which had already passed its peak, ended in days. 

Though it took years for Snow’s theory to achieve widespread acceptance, his approach is central to modern epidemiology. Investigating the source of outbreaks can prevent new cases, but it also gives us a better understanding of diseases and helps manage public fear. Even when infections have stopped, outbreak investigations are useful to develop strategies for preventing — and, failing that, responding to — future outbreaks. 

Two recent outbreaks have demonstrated the necessity — and the challenges — of such investigations, almost two centuries after Snow’s pioneering work. The first was the hantavirus outbreak that dominated headlines last month. Then, on May 17, the World Health Organization (WHO) declared a public health emergency of international concern, the highest level of global health alert, in response to an outbreak of the deadly hemorrhagic disease Ebola in the Democratic Republic of the Congo (DRC), which, as of June 2, had killed 62 people, with 363 confirmed cases. It’s the 17th Ebola outbreak in the DRC and one of the largest on record. It has spread to neighboring Uganda, where, as of June 4, there are 16 confirmed cases, one confirmed death, and one probable case and likely death. 

The first confirmed case, a healthcare worker in Bunia, DRC, died on April 24, but the outbreak may have been spreading undetected since as early as January. Investigators haven’t identified patient zero — the index case — and still don’t know how this outbreak began. Abdou Sebushishe, a doctor working with the International Medical Corps in Goma, DRC, told CBS News that up to 20 percent of current patients are themselves healthcare workers. He estimated that it may be more than six months before the outbreak could be controlled, given that the disease is outpacing the current response.

Part of the challenge is that the current outbreak is caused by the Bundibugyo strain of Ebola, which is relatively uncommon and has a genome about 30 percent different from the Ebola viruses that usually spark outbreaks. Testing for more common variants didn’t pick up the Bundibugyo virus right away, and ongoing conflict in the DRC contributed to the delay and continues to make contact tracing difficult. Unlike other strains, the Bundibugyo virus has no approved therapeutics or vaccines.  

In the past, researchers have had some success identifying the index case of Ebola outbreaks. Investigators managed to identify the first patient of the 2014-2016 West Africa Ebola epidemic — the largest and deadliest in history, with more than 15,000 confirmed cases and 11,000 deaths — as a toddler in the west African nation of Guinea. What’s harder to definitively determine is how the boy, who died in December 2013 before the outbreak had been identified, contracted it. It’s possible that he came into contact with an Ebola-infected fruit bat or its droppings while playing in a hollow tree, but scientists can’t say for sure.

Investigating outbreak origins is inherently fraught and can lead to the international fingerpointing that characterized much of the Covid-19 pandemic. But it’s not primarily about assigning blame. Instead, knowing where and how outbreaks began informs how we respond to them, halt transmission, communicate to the public, and prevent them from happening again. It can identify high-risk regions and influence how public health officials monitor a disease. As the recent Ebola and hantavirus outbreaks demonstrate, however, that effort is often complicated by a host of factors, and the resulting uncertainty makes it that much harder to manage public health concerns efficiently and well. 

The curious case of Legionnaires’ disease in New York City

Our epidemiological tools have come a long way since John Snow used hand-drawn maps to identify the source of the Soho cholera outbreak. The value of these new tools lies in the information they generate — which is crucial to fighting outbreaks. 

Take the case of New York City’s biggest — and deadliest — outbreak of Legionnaires’ disease (LD), a bacterial infection that causes a severe pneumonia and has a fatality rate of 10 percent. By the time public health investigators detected it in the summer of 2015, dozens had already been hospitalized. It was the second-largest LD outbreak in US history, infecting 138 people and killing 16. 

The initial epidemiologic investigation started with contact tracing to find the source of the disease, but the results didn’t suggest any shared exposures. Cooling towers, which provide water for air conditioning systems in the form of an inhalable mist, had been involved in previous LD outbreaks, but officials didn’t know how many cooling towers there were in the city or how well-maintained they were. 

Investigators ultimately located and tested 55 cooling towers in the South Bronx, where cases were clustered, for Legionella. They identified the source: a single cooling tower atop the Opera House Hotel. The hotel disinfected the tower, and New York’s City Council passed new regulations requiring every building in the city with a cooling tower to register it with the health department, test it every 90 days, and remediate it if Legionella was found. 

Within a year, the health department inspected almost 80 percent of the city’s towers — detection and disinfection that would have never been conducted otherwise. No large LD outbreaks emerged — until inspections declined in 2025. “Regulations do not enforce themselves,” Jay Varma, a physician and epidemiologist who served as incident manager for the 2015 New York outbreak, wrote last year in Healthbeat. “The Covid pandemic has sparked a strong backlash against government authority, and austerity budgets are now starving public health agencies. Infections may be inevitable, but outbreaks are a choice.”

Cholera and LD are waterborne, but Ebola and hantavirus, which first cross over to humans from animal reservoirs, present a different challenge. 

The challenge of hantavirus and Ebola

“The end of the world, the beginning of everything” is the motto of Ushuaia, Argentina, the southernmost city on the planet, where tourists flock to watch birds and embark on cruise ships. It’s the main gateway to Antarctica, making up 90 percent of all cruise departures to the continent. 

It’s here that a Dutch couple may have contracted the Andes virus, the only strain of hantavirus known to spread from person to person, before sparking an outbreak on the MV Hondius. The Argentinian government’s prevailing theory is that the couple got infected while birdwatching at a landfill in Ushuaia before the cruise, coming into contact with the rodents that carry the Andes strain. 

Well, maybe not. 

“The current theory of a couple birdwatching in southern Argentina may not be plausible, because the [long-tailed pygmy] rice rat that is responsible for spreading the Andes strain of the virus is usually found in northern Argentina or Chile, and we know the birdwatching at the landfill occurred in the southern part of Argentina,” Omer Awan, a physician and public health expert, told me over email. There have been no recorded cases of hantavirus in Tierra del Fuego province, where Ushuaia is located, before. 

“Understanding the origins of the outbreak will be helpful in guiding interventions like rodent control, isolation protocols, and…how the rare Andes strain of Hantavirus is transmitted,” Awan said. “[And] identifying the source of the [2026] ebola outbreak can influence response strategy and how public health officials monitor the virus.”

Delayed detection and human movement — especially for illnesses like hantavirus and Ebola that can incubate over the course of weeks — make tracing the source of an outbreak difficult, even in the best of circumstances. We still don’t know the original source of the first Ebola outbreak in 1976, which occurred in two simultaneous waves. Debates still rage over whether Covid-19 emerged naturally through zoonotic spillover — the virus jumping from an animal host to humans — or if it potentially escaped from a lab in an accident. We know that the hantavirus and Ebola outbreaks are natural in origin, but there are still international efforts to shift the “blame” from Argentina to neighboring Chile, especially with economic interests on the line.

Such spillover events have only become more likely as humans destroy ecosystems and infringe on animal habitats. Climate change exacerbates existing infectious disease risk. “Because of our choices as a society, there’s a one-in-five chance that another pandemic will occur in the next decade that will kill at least 25 million people,” Neil Vora, the executive director of Preventing Pandemics at the Source coalition, wrote in Time Magazine. 

Determining the source of outbreaks is even more difficult — and politically perilous — in the post-Covid era. The US and Argentina have pulled out of WHO. Global health funding cuts, on the part of the US as well as other countries, have weakened our biosurveillance architecture and ability to effectively respond to infectious disease. 

Compared to Covid, the scale of the 2026 Bundibugyo and hantavirus outbreaks are small. It’s still proving hard to get answers. That’s going to be a serious problem whenever the next pandemic arrives — and it is a matter of when, not if. 

An evolving threat landscape

Although we face escalating spillover risks from habitat destruction and climate change, we can’t count on the next global infectious disease threat being naturally occurring in origin when it does come. 

“It’s very clear that artificial intelligence capabilities are advancing incredibly rapidly,” Jaime Yassif, senior advisor for global biological policy and programs at the Nuclear Threat Initiative (NTI), told me. “[That could] make it easier for novice actors to engineer pathogens that we [already] know about or for sophisticated actors to engineer novel pathogens that are more dangerous than what’s found in nature.”

If there is an outbreak of uncertain origin — where it’s unclear if it’s natural, accidental, or deliberate — we lack robust international mechanisms that can investigate the source and quickly arrive at a conclusion. That would make it harder to address the source proactively, whether that means stopping future natural spillover events, preventing lab accidents, or holding bad actors to account. 

Public health professionals would need to take additional precautions if there was a risk of a deliberate outbreak, as we saw with the 2001 anthrax attacks, where letters laced with Bacillus anthracis were sent in the mail, infecting 17 people and killing five. A naturally-occurring anthrax exposure would have required a different response, since a bioterrorism investigation has to contend with the additional challenge of determining criminal responsibility. 

And as we’ve seen with the debates around Covid-19 origins, suspicion that something was caused by human activity can be incredibly corrosive to international trust, making necessary geopolitical cooperation in the face of outbreaks significantly harder. 

NTI identified that preparedness gap and proposed a Joint Assessment Mechanism to identify the source of outbreaks of uncertain origin. It would be housed in the UN Secretary-General’s Mechanism for Investigation of Alleged Use of Chemical and Biological Weapons (UNSGM) in order to pull together different components of the UN system and bridge security and public health. 

That project (which I supported and advocated when I worked at NTI from 2022 to 2024) is currently on pause. “We still think it’s a vital gap and really important, but we just couldn’t get the political will to move it forward in the system, notwithstanding the significant support for it internationally in various quarters,” Yassif said.

We are simply unprepared domestically and internationally to prevent, detect, and respond to global infectious disease threats. Emerging infectious disease outbreaks threaten us all, and we are nowhere near where we should be in order to protect vulnerable populations and countries around the world. While the current Ebola and hantavirus outbreaks are very unlikely to become pandemics on the scale of Covid-19, they’re still dangerous and deadly. Unless we can determine where and how they began, we’ll be ill-equipped to stop them from recurring. And next time, things could be far worse.

Shortly after brandishing his infamous chainsaw on a conservative conference stage last February, Elon Musk attended a Cabinet meeting where, giggling slyly, he admitted to having “accidentally canceled” Ebola prevention in his haste to obliterate the US Agency for International Development (USAID).

“We restored the Ebola prevention immediately,” he added coolly at the time, “and there was no interruption.” That claim has since proven to be disastrously, profoundly untrue. 

On May 17, the World Health Organization declared a rapidly spreading Ebola outbreak in the Democratic Republic of the Congo and Uganda a “public health emergency of international concern,” only the ninth-ever time the agency has made that designation. In the weeks since, at least 220 people have died of the highly fatal virus, and more than 900 suspected cases have been identified so far. It is already the third-largest Ebola outbreak on record.

And yet, that toll is likely a tremendous undercount because, as the New York Times reported from the ground this week, “only a trickle of tests are being processed every day” in the cities most affected by the outbreak. “The virus is far ahead of us,” Ahmed Mahat, a manager with International Medical Corps, told the Times. “And it’s spreading fast.”

In fact, publicly known cases are rising exponentially faster than in any prior outbreak, including the largest ever, West Africa’s catastrophic outbreak in 2014, and the second-largest in 2018. By the time this outbreak was declared, hundreds of people had already been infected.

When you stop looking, you can’t see

Why did this outbreak spread so quickly? Part of it was the virus itself, a rare Bundibugyo strain of Ebola, which is harder to diagnose and for which there are no vaccines or treatments. (At least, not yet.) Another reason is that this outbreak began in a remote province of eastern Congo, an active war zone, where what health systems exist have been ravaged by decades of armed conflict. 

As if the odds weren’t already stacked enough, however, this outbreak broke out under the heavy shadow of US foreign aid cuts that, among other calamities, gutted the world’s Ebola detection and response apparatus last year. Despite Musk’s earlier assurances, US-funded programs to detect new Ebola cases and dispatch a response were indeed frozen under the Trump administration, according to Stat. US cuts also indirectly contributed to the outbreak by weakening local health systems and stockpiles. 

Altogether, the US Department of Health and Human Services disbursed about $10 million to Congo last year, down from $33 million the year prior, Stat noted. USAID sent $693 million in aid to Congo last year, down from nearly $1.2 billion in 2024. 

Cuts to disease surveillance meant that this virus took longer to identify than it should have. And with cuts to local health systems, it’s now much harder to come by the tests, nurses, doctors, and protective equipment needed to stop the spread. 

“It’s so bad. It’s so bad,” Jean Kaseya, director-general of the Africa Centre for Disease Control and Prevention, told Devex. The Africa CDC’s role in quelling outbreaks has become even more important as wealthy countries have retreated from the global health stage, but it is impossible to fill all of the medical surveillance gaps left by the US withdrawal of support, he said. “No one can give you the magnitude of this outbreak.”

Bleeding out

The US has done some course correction since the outbreak began. Last week, the State Department pledged $23 million in emergency funding for Congo and Uganda, plus the deployment of a disaster response team and enhanced involvement from the CDC, which says it’s been actively coordinating with local health agencies. At least some lost funding should have also begun flowing back to both countries through their bilateral aid deals with the US. 

But when you lose a limb to a chainsaw — even a “chainsaw of bureaucracy” like the one Musk dragged across a stage — you can’t expect a bandaid to make up for the damage. Beyond the money, the US withdrawal from the WHO and other policy decisions have had a deeply destabilizing effect on global health systems, which no doubt helped bungle this outbreak response. In many cases, the disease experts and researchers who were once in charge are simply not there anymore. 

Given the outbreak’s virulence so far, things will probably get significantly worse before they get better. While the majority of cases have occurred in Congo so far, Robert Redfield, former head of the CDC, predicted last week that the virus could soon spread to neighboring countries like Tanzania and South Sudan. Researchers have rapidly begun development on a new vaccine for the deadly virus, but even in a very best-case scenario, it will take months to roll out. In the meantime, health workers will continue to play catch-up to a virus that now has a massive head start.

As Nicholas Enrich, the former top global health official for USAID, told the New York Times last week: “In a time when hours matter, we’re delayed by weeks.”

As global concern about an Ebola outbreak in central Africa grows, hundreds of Kenyans have taken to the streets to protest a plan by the Trump administration to send American citizens who have been exposed to the virus to Kenya, rather than bringing them back to the US. Two people have been shot and killed during the protests. 

The outbreak started in the Democratic Republic of Congo last month and has since spread to Uganda. There are currently no confirmed cases in Kenya, which shares a border with Uganda.

Kenyans are demanding to know why the US wants to send Ebola patients to their country, and why their government gave the US the initial approval to build a 50-bed quarantine facility at the Laikipia Air Base in central Kenya.

For now, the plan is on hold after a court ruling in Kenya; on Tuesday, the court extended the suspension to at least June 23 and also ordered the Kenyan government to provide details of its arrangement with the Trump administration, including financial agreements and measures put in place to protect Kenyans.

Between cuts to American foreign aid in the region, the sheer aggressiveness of this strain of the virus, and conspiracy theories that threaten public health workers, many public health workers fear that this Ebola outbreak has become a perfect storm.

To understand what’s going on — and why the US is trying to involve Kenya — Today, Explained co-host Noel King spoke to Sabrina Siddiqui, a national politics reporter for the Wall Street Journal who helped to break the story. They discussed the reactions from Kenyans and public health experts and what would happen if Kenya continues to rebuff the administration.

Below is an excerpt of their conversation, edited for length and clarity. There’s much more in the full podcast, so listen to Today, Explained wherever you get podcasts, including Apple Podcasts, Pandora, and Spotify.

What is the plan?

The administration has been trying to set up a quarantine facility in Kenya at an air force base where they would essentially house Americans who have been exposed to Ebola and anyone who also tests positive.

They’re describing it as somewhat of a tent hospital. But there are various plans underway for also adding, if needed, isolation units and biocontainment units. That is, of course, if there are people who truly get sick or need further care. 

I think they see this as an opportunity to have a place for Americans to quarantine while they’re evaluated, and they have deployed public health officers from the United States to assist with these efforts. They have also said that if Americans test positive, they would only perhaps stay at this facility for a couple days before being sent to another country. And they’re looking at facilities in Europe that could potentially accommodate Americans if they were to truly get sick.

What the US is saying is: We don’t want you coming back into the US. You look at the reaction to this here at home, and there’s a lot of shock. Ebola outbreaks have happened before. This is a very dangerous, dangerous virus. How does the US usually handle this when our citizens are affected?

That’s actually been very striking about the administration’s response to this particular outbreak. In previous outbreaks, Americans who had been exposed to Ebola or who had tested positive were allowed to return home and they were monitored and cared for at quarantine facilities here in the United States. And we do have biocontainment units as well. During this recent hantavirus outbreak, American passengers who were aboard the cruise ship where that outbreak occurred have been quarantining at one of those biocontainment units in Nebraska. 

So it’s frankly been bizarre to a lot of public health officials and epidemiologists that Americans would not be allowed to come home. And it just appears to be the case that the Trump administration is taking a very hard line against letting anyone who is known to have Ebola to be allowed back here in the United States. What they’re saying is that they do not want any Ebola cases to exist in the United States during this outbreak.

So the plan is: send Americans to Kenya. And what is the status of that plan?

The Trump administration announced that the US and Kenya had reached an agreement to stand up this quarantine facility for Americans in Kenya. And then a Kenyan high court put a temporary hold on the Trump administration’s plan to set up that facility. So right now, the plan is very much in limbo. As of now, it’s not clear if the plan is even going to move forward.

How did people in Kenya respond when they were told the United States wants to send its citizens to you?

One of the lawyers who is part of the legal group that is arguing this case said, “Is Kenya being reduced to a dumping site?” I think that really captures the mood of many Kenyans who learned about this plan through news reports, and were critical of their government for agreeing to allow Americans who had been exposed to Ebola to be rerouted to Kenya when there are no known or suspected cases of Ebola in Kenya. 

There are obviously a lot of concerns, including from medical groups in Kenya that there could perhaps be an outbreak in Kenya that stems from bringing Americans to the country who’ve been exposed to the virus.

Does anyone know why [the administration chose] Kenya?

The administration said that they were looking for somewhere in the region that is unaffected by the outbreak, where they don’t believe there is as high a risk of spread and that is not too far so that people could get there quickly. Obviously there are also politics involved and it seems like they were able to come to some kind of agreement with the government, even if it’s been halted by the courts. 

Again, this is temporary for people who actually get sick. So it doesn’t even look like it was necessarily a long-term plan in terms of how they plan to actually use this facility, because at the same time that they’re saying Americans can quarantine in Kenya, they also said that anyone who truly gets sick would be evacuated to a tertiary care center and that they’re currently talking to partners in Europe to try and identify where sick patients can be taken. 

These are just some of the questions that a lot of people have around the administration’s plans, which they haven’t been terribly forthcoming about, and which have drawn criticism not just from people in Kenya, but also from public health experts here at home who simply do not understand why they would not allow Americans to return to their home country.

Let me ask you what you’ve been hearing from public health experts, because there is, from the non-expert’s point of view, a knee-jerk sense in this. It’s: Ebola is dangerous, keep people where they are, or keep people elsewhere, so that they don’t bring Ebola into the United States. 

You said public health experts say this does not make sense. Why doesn’t it make sense? What do they tell you?

I think there are a couple of things that are at play. One is that public health experts do say that it is the responsibility of the United States government to take care of its own people and to allow them to return home so that they could receive the highest quality of care and that they have these state-of-the-art facilities specifically designed for outbreaks and viruses like Ebola.

I also think that there is the component of mental health, and that, in addition to just needing to receive the appropriate care, that people should have access to their support system, that they should be allowed to be in closer proximity to their families if they were to get sick. And people see that as a moral responsibility that the United States has to afford Americans that opportunity. 

There’s also just the fact that in previous outbreaks, Americans were brought home, and the Trump administration has not provided a medical rationale for why they’re so opposed to Americans coming back home other than saying that time is of the essence when someone has Ebola. Well, time was also of the essence in prior outbreaks, and the US did not stop Americans from returning home.

You’ve been covering the hantavirus outbreak as well. And I wonder whether you’re seeing a pattern here in the way this administration is responding to these public health crises where the public is inclined to freak out a bit and public health experts might have a different idea of what needs to happen.

Well, here’s what’s really fascinating about covering the hantavirus outbreak as well as the Ebola outbreak. The Trump administration has been willing to embrace these very aggressive quarantine and isolation measures despite the fact that this administration is full of people at the highest levels of leadership who were so critical of what they saw as heavy-handed social distancing and isolation guidelines during the Covid-19 pandemic.

And they’re going even further. There were a couple of passengers who wanted to leave the Nebraska facility where those who’ve been exposed to hantavirus have been quarantining. And the acting director of the CDC, Jay Bhattacharya, signed an order forcing them to stay there. And now, as those passengers are reaching the end of their quarantine period — these are those who are exposed to hantavirus, who have been asymptomatic and do not have hantavirus — they’re now returning to their home states. The Trump administration is essentially insisting on 24/7 monitoring and not allowing them to leave their homes.

So, oddly enough, it’s a very heavy-handed way that the Trump administration has responded to these outbreaks, even though they were the ones who used criticisms of public health institutions and of the scientific community during Covid as a way to appeal to voters who are frustrated by these exact kinds of guidelines and rules during that pandemic.

What are the stakes here? What happens if Kenya says, no, President Donald Trump, we’re just not going to allow this?

Well, that’s actually going to be a really interesting moment if it comes to pass because it is not entirely clear if the Trump administration has a plan B.

It just seems like this entire plan came together very quickly. Even the public health officers who were deployed to Kenya when they were called upon for this assignment only received about three days of training. And that’s something that some public health officials said simply isn’t enough for people who are going to go and try to staff a facility where you have this rare strain of a deadly virus. 

When the Trump administration is talking about whether or not they would be able to send Americans to other facilities in Europe, they still haven’t identified where those care centers would be, which just signals that they haven’t really thought through what would happen if they are not allowed to stand up this facility in Kenya. And I suspect that while they’re still negotiating with the Europeans, it’s very likely that people in Europe would have the same reaction as those in Kenya: “Why are you sending potentially sick Americans here rather than allowing them to return home?”