Measles, a dangerous illness that for decades has rarely infected Canadians, is back – and spreading.

As thousands of people in the country get sick, we ask: How have cities and towns across the world once again become a fertile ground for measles outbreaks? And where was the spark that ignited North America's viral blaze?

This is the story of The Globe and Mail's 40,000-kilometre journey to find out.

No one saw much cause for alarm when the Pattanasat Witaya School in the deep south of Thailand first noticed last summer that children were getting sick, their chests flushing with bumpy red spots.

“It started small,” recalled Mustafa Durrah, the director of the school, a private institution that offers Islamic instruction in a part of the country where mosques are more common than temples. A handful of students became ill, displaying a tell-tale rash that most in this part of the world can still easily identify.

It was the measles, that pestilence that has defied one of humanity’s most intense eradication efforts, once again tearing through the bottom of Thailand. Over the course of months, it infected thousands in the country and killed nearly a dozen.

The outbreak posed no great enigma to doctors, imams and school principals here, because they have come to expect this sort of thing every half-decade. In a place where large numbers of children are not vaccinated, that’s roughly how long it takes to build up a sufficiently large unprotected population for the virus to once again feast uninterrupted as it passes, breath by breath, through vulnerable human hosts.

Most recover, after experiencing rashes, fevers and days when exposure to light can be painful. But some die. As recently as the 1970s, measles killed 2.6 million people a year around the world.

At Pattanasat Witaya School, roughly one in five students had not been vaccinated. Parents in the area refused the jab for a raft of reasons. Some feared – wrongly – that it contains pork products, violating halal guidelines. Others worried post-vaccine fevers in children would keep them from work. Still others saw the tiny glass vials of weakened measles virus as a disguised weapon, designed by religious enemies to bring about their demise.

Whatever the reason, what started as two or three sick students quickly became nearly 100. The school closed its doors July 26 to disinfect.

“It spread really fast,” Mr. Durrah says.

He could not have known just how fast, or how far.

He could not have expected that embers of the viral blaze that passed through the place he lives and works would be carried across an ocean by the winds of high-altitude travel, vaccine skepticism and the most intimate family unions – including nuptials on the opposite side of the planet, in rural New Brunswick.

The outbreak has been so widespread in North America that those who study public health are no longer confident Canada – and perhaps the U.S. with it – will be able to maintain its measles elimination status, which requires putting a halt to continuous transmission of the virus. Canada first eliminated the measles 27 years ago.

The return of measles marks a consequential moment, an epidemiological echo of broader change convulsing society.

Erosion of confidence in traditional sources of authority has altered medicine as it has politics, amid a ferment of cultural skepticism that pays little heed to condemnation from the establishment. It is no coincidence that the measles re-emerged in the long shadow of the COVID-19 pandemic.

Yet little is known about this outbreak’s origin, or the route it travelled, details that have eluded some public health authorities and been obscured by others.

It all begs an important question.

How did the distant corners of Earth once again find themselves woven together by this sickness?

Behind my search for the source of the measles resurgence

The answer can suggest ways to prepare for future outbreaks. It also promises to show us more vividly how complacency and doubt have slowed the march of medical technology, even as travel in the jet age continues to speed ahead.

When cases began to emerge in New Brunswick last year, Canadian authorities reported an initial travel-related infection that, they said, arrived on a flight from Manila.

Soon, outbreaks began to arise across North America in volleys of viral fireworks that burst across the continent, through dozens of states and provinces and into northern Mexico.

Four people have died in Canada and the U.S., with an additional 21 deaths in Mexico.

Cases bearing a similar genotype – the measles strain known as D8 – were reported by public health authorities as far away as Belize, Argentina, Bolivia, Brazil and Paraguay.

How, exactly, did this happen?

To trace the roots of these outbreaks, The Globe and Mail travelled more than 40,000 kilometres, to the mosque-speckled horizons of southern Thailand, to a rural New Brunswick community famous for its annual tractor pull and to the plains of western Texas, where pumpjacks do their lumbering work next to restaurants that serve cheese curd-filled wrenakje Mennonite perogies.

What we found were some of the first shoots that would become the tangled undergrowth of an outbreak the length of the Americas, including five critical days when it could possibly have been cut short.

‘I have no idea how it got here’

From the very beginning, the measles outbreak in Seminole, Tex., presented a puzzle. For the hospital staff who saw the first recorded patient, it wasn’t even clear what was ailing the girl who came into the emergency room in late January with a rash, at the outset of what would become the most extensive U.S. outbreak in the past year.

Like a great many of her colleagues in Canada and the U.S., Dr. Leila Myrick had never seen a case of the measles. She had to resort to medical texts to help discern what she was seeing, a child sick with a disease that was, in generations past, among humanity’s most reliable killers. An estimated 200 million people died from the disease between 1855 and 2005, a number equivalent to the modern population of western Europe.

With the measles successfully eliminated from 82 countries, however, the virus has vanished from view for much of the medical profession. In 2024, the Centers for Disease Control and Prevention reported just 285 cases across the U.S., where a disease is considered rare if it affects fewer than 200,000 people.

When the measles did appear, public health workers could usually trace a clear lineage. In 2019, only one of six small outbreaks in California could not be linked to a specific person with a recent history of international travel.

But the family who brought their sick girl to the Seminole Hospital District early this year had no such information.

“They really didn’t tell me much,” Dr. Myrick says. “They were very private and didn’t speak good English. So all I know is that they are from Seminole.”

In mid-January, Houston had reported two people with measles, both of whom had recently been outside the country. The girl in Seminole was confirmed as a measles case on Jan. 29.

But her family reported no travel. They made no mention of a visit from anyone outside of western Texas, where Seminole is the administrative seat for Gaines County, a wind-lashed corner of the southern High Plains where dust devils swirl across fields of peanuts and cotton.

“I have no idea how it got here,” says Dr. Myrick.

In the weeks that followed, measles tore through the area, with hundreds of confirmed cases. U.S. Health and Human Services Secretary Robert F. Kennedy Jr. paid a personal visit in April, saying he had come to console two families who had lost children during the outbreak, but also to learn how federal agencies could better partner with Texas health officials “to control the measles outbreak.”

But neither he nor those local officials could ever say how measles arrived in Texas.

“I don’t think in Gaines County they’re ever going to know who the index case was,” says Katherine Wells, the director of public health in the nearby city of Lubbock.

She did, however, have a guess.

“I’ve always suspected that the measles came from Canada,” she says.

Her supposition was founded on historical ground. Western Texas may be geographically distant from Canada, but the past has lashed the two places together.

In 1977, more than 100 Mennonite families moved to the Seminole area from Canada and Mexico, in search of land and a place they could call their own – a place to educate their children and live as they chose, keeping alive Anabaptist traditions and their Plautdietsch language.

Helen Friesen was four that year. Born in Tillsonburg, Ont., her family was among the first to move to Seminole, bringing with them habits from home. Today, Ms. Friesen’s house is surrounded with live oaks and juniper trees, their canopies offering a better respite from the glaring sun than the low-lying mesquite that naturally covers the landscape.

“We like our trees because in Canada we had trees,” she says. “It’s like trying to get the desert to bloom.”

For those first Mennonite families, the transition to Texas was not easy. Many came as tourists and after three months the U.S. government declared them illegal aliens, ordering them out. It took an act of Congress to grant the community citizenship. Even then, it took decades of work to secure economic prosperity after the first arrivals discovered that much of the land they purchased did not include water rights.

“You can see why some of the people are a little hesitant to believe everything authorities say,” Ms. Friesen says.

When the measles began to spread early this year, many had little interest in reaching out to hospitals or public health officials. They, like Ms. Friesen, took matters into their own hands. Two of her children got sick with the measles. All of her grandchildren did, too.

“You treat it like a virus. You just don’t overthink it,” she says. Some Mennonite families vaccinated. Others did not, out of concern over side effects, or because they believe measles does not pose a serious health risk – or simply from a desire to use health products considered more natural.

When Ms. Friesen needed additional guidance, she turned not to doctors but to Prescription for Nutritional Healing, a manual originally published in the 1980s by a woman who began writing nutritional advice after closing the Indianapolis nightclub she ran.

The book counsels catnip tea and garlic enemas to reduce fever, as well as vitamins, zinc lozenges and proteolytic enzymes. But it also provides a detailed description of the seriousness of a measles infection, which can develop into more dire medical problems including encephalitis, inflammation of the brain – and describes the preventive benefits of the measles vaccine.

In her home measles care, Ms. Friesen rubbed lobelia on congested chests, administered olive leaf spray for sore throats and prescribed large quantities of garlic. She and others doled out plenty of cod liver oil, too, favoured as a source of vitamin A, which the World Health Organization says may reduce measles deaths when given to sick children and adults.

At one point, local supplies ran so low that Willy Bergen, a corporate pilot from the area, was called for help. A family’s infant twins were very sick and in urgent need.

“Those kids were in bad shape,” Mr. Bergen says. But the family was “scared to go to the hospital because of how some people were being treated.” Stories quickly circulated about sick people being turned away by medical providers reluctant to see measles patients.

Mr. Bergen flew more than 850 kilometres to Scottsdale, Ariz., where he loaded a King Air 350 with cod liver oil and vitamins to wing back home.

Among the sick were Mr. Bergen’s four children. Measles ripped through the school they attended, too, a small community-run institution where Mr. Bergen once taught and now sits on the board. They shut down briefly in mid-March. Others were affected much earlier. The Gaines County Mennonite School closed for the first week of February, just days after the first confirmed case.

What’s not clear, however, is how the virus got there.

It is “very, very difficult to trace,” says the school’s principal, John Wiebe.

For local authorities, that difficulty was elevated by a lack of information.

Local Mennonite schools are privately run and under no obligation to report basic public health information – such as the vaccination status of students. Zach Holbrooks, executive director for the South Plains Public Health District, heard about closures second-hand; he wasn’t even aware some of the community’s small schools existed. Those families who did seek treatment tended to offer little information.

Pandemic memories remained fresh in the minds of people who balked at government interventions and grew even more skeptical of authorities they already felt reason to distrust, given their community’s history.

When contact tracers sought to track measles cases, they often heard, “No. I’m not going to tell you who I’ve been in contact with. I’m not going to give you the names and numbers,” Mr. Holbrooks recalled.

“COVID ruined it,” he says. But “all you can do is ask. We have no police power.”

The county resorted to distributing flyers, buying ads on YouTube and dispatching a box truck to display messaging. It tracked the virus spread by testing wastewater.

None of that did anything to answer the question of how the measles came to Texas.

“How it got to Seminole? I have no idea,” Mr. Holbrooks says.

In the months following the Texas outbreak, one important clue to its origins came from public health researchers, who tracked the spread across the U.S. of one dominant viral lineage they call “MVs/Ontario.CAN/47.24,” a name that points to its origin north of the border.

The outbreak in the U.S. “absolutely came down from Canada,” says Joanne McGovern, a lecturer at the Yale School of Public Health who has worked with students there to conduct weekly tracking of the global outbreak. Cases in Mexico, she added, share that common lineage.

But none of the officials managing or monitoring the measles had access to the reports that had filtered in to one small business owner in Seminole.

For months, families who refused to go to hospitals or speak with contact tracers sought help from someone they did trust. Nancy Ginter owns Health 2 U, a local store stocked high with alternative products: essential oil drops, chocolate bars without added sugar, organic fruit pops, immune boosters, turmeric powders.

Ms. Ginter was born in Leamington, Ont., and moved to Texas with her family at the age of 11. She began selling health products from her home in the 1980s, turning a personal interest in wellness into a thriving business that now occupies a large retail store. Customers come from hundreds of kilometres away.

As the measles spread, her store became an important centre for those seeking counsel and remedies.

“We’ve seen about 200 people a day,” Ms. Ginter says. “It went on for months.”

It began before health authorities had any idea that an outbreak was on its way.

Days before the first confirmed measles case, Ms. Ginter’s phone began to ping with photos from parents wondering what to do. They sent pictures of children with chests covered in rashes. “They didn’t know what it was at first,” she says.

Ms. Ginter, however, was prepared. In the first week of January, she was in her store when two women came in. One had driven from her home 120 kilometres away with an urgent question.

“They had gotten company over Christmas,” Ms. Ginter recalled. After the visitors arrived in Texas, they received notice that they been exposed to the measles.

The visitors “left because they didn’t feel good,” the women told Ms. Ginter.

“They asked me what they could do, and I said, ‘I don’t really know. We haven’t had something like that in a long, long time,’” Ms. Ginter recalled. She recommended supplements to boost their immune system. After the women left, she chatted with her own family about what might be coming.

“I told my daughters and said, ‘I don’t know if we need to prepare ourselves, because I had just heard of somebody that might get measles,’” she recalled.

At that moment, she had no idea what was to come. But the women did tell her one thing about their sick visitors.

They had come from Canada.

The vaccine and the unvaccinated

Among the many reasons to fear the measles, one in particular stands out. It is capable of spreading through human populations like few other viruses. The R0, or r-naught, of a pathogen, describes how many others can be infected by a single person – a once-obscure medical term made popular by COVID.

The R0 for the flu is between one and two. Ebola is two. Smallpox is five to seven, chickenpox nine to 10.

For measles, it is 12 to 18. It can linger in the air for hours and boasts an extraordinary ability to sicken anyone it contacts.

“If you are exposed and you are not immune, the chance of your actually getting infected is tremendously high. Some people say as much as 90 per cent,” says Megan Ranney, an emergency physician who is dean of the Yale School of Public Health.

Small wonder, then, that measles has been an inescapable part of human existence for at least a thousand years, first described in detail by Rhazes, a polymath Persian physician who lived in the 9th and 10th centuries. In a treatise, he noted that in hot years, when “the rains come on very late, then the Measles quickly seize those who are disposed to them.”

Genetic analysis traces the emergence of the measles virus to the 6th century BCE, around the time human cities reached a size big enough to allow it to prosper. So successful was it that even among epidemiologists, it was once considered impossible to dislodge from the human experience without altering some fundamental element of biological balance.

Some believed “we must learn to live with this parasite rather than hope to eradicate it,” Alexander Langmuir, chief epidemiologist for the Centers for Disease Control and Prevention from 1949 to 1970, wrote with several colleagues in 1962.

But, they added, “Any parent who has seen his small child suffer even for a few days with persistent fever of 105 degrees, with hacking cough and delirium wants to see this prevented, if it can be done safely.”

They wrote at a momentous time.

In 1954, the same year Boston Children’s Medical Center researcher John Enders won the Nobel Prize for work on the polio virus, one of his research colleagues gathered throat swabs and blood samples from children at a boarding school near Boston in the throes of a measles outbreak.

Over the ensuing years, they isolated the virus, then propagated it numerous times through different types of human cells and chicken embryos. Each passage through those cells weakened the ability of the virus to sicken humans until it reached a point that, when injected in a child, it would prompt an immune response without causing severe illness.

The attenuated virus, named after the 13-year-old schoolboy from which it was drawn, David Edmonston, became the basis for a vaccine. It was licenced for general use in 1963 and, through decades of effort, was brought to the farthest reaches of earth.

A parallel effort made rinderpest, a deadly cattle infection caused by a virus that is the measles’ closest biological relative, the first disease eliminated from animals in 2011.

Writing in 1962, Dr. Langmuir and his colleagues foresaw the same for the measles.

“Its eradication from large continental land masses such as North America and many other parts of the world can be anticipated soon.”

But animals have proven easier creatures to immunize than humans. Rinderpest is gone. Measles remains – and is resurgent.

What Dr. Langmuir could not have foreseen was how the vaccine could be rejected by those resisting political disenfranchisement, swayed by conspiracy theories or afflicted with the very human tendency to forget the worst elements of the past.

In Canada, the number of young children with at least one dose of the measles vaccine fell from 90 per cent in 2019 to below 83 per cent four years later. In some smaller communities, less than a third of children received the vaccine.

For epidemiologists, watching those rates decline across North America has been like observing a landscape that grows increasingly dry from years without sufficient precipitation. “When you throw a match out of the window into an area that hasn’t had rain in a while, eventually it’s going to start a wildfire,” says Dr. Ranney, the Yale scholar.

What perhaps no one could have expected is that the first flames of conflagration would erupt in rural New Brunswick.

The ‘best day’ and ‘worst day’

Charity Driedger had good reason to think crisis would attend her wedding. It wasn’t so much a premonition as family history. The youngest of nine, she had watched as a procession of older siblings navigated unexpected dramas as they tied the knot.

So when the day came and her photographer failed to materialize on time, she figured that was it. The streak of misfortune had returned.

“Little did I know,” she says.

It was Oct. 18, 2024. She married Josiah Driedger, in a ceremony with a guest list of 167. The wedding took place in Florenceville, a small community in eastern New Brunswick’s St. John River Valley that is the global headquarters of McCain Foods.

Ms. Driedger’s family, originally from Quebec, moved to New Brunswick in 1992, then to Ontario before returning again to settle in New Brunswick. They built homes and lives outside of Centreville, a place best known for its high-octane truck and tractor power pull.

The wedding was a joyous affair, with friends and family from Maine, Massachusetts and Tennessee. Most of those who gathered came from Ontario, where Mr. Driedger is from, and where Ms. Driedger had lived for 14 years. Others came from much farther. The midwife who was at Ms. Driedger’s birth arrived from Scotland. Ms. Driedger’s older sister, Lea, made the longest trek, flying in from Thailand with her 12-year-old son.

As the time for the ceremony approached, a panicked phone call brought the photographer hustling to the church and the wedding proceeded after a small delay. In addition to the traditional bridesmaids and groomsmen, the couple asked nearly 20 people to join them in similar attire, in a celebration of love and community.

“We called it the cloud of witnesses, because we both have lots of friends and we just wanted them to be a part of our day,” Ms. Driedger says. “Lea was a part of that.”

She was vaguely aware that her sister wasn’t feeling well, but figured it must be jet lag, the exhaustion of someone who had travelled from Asia. Besides, any concern was overwhelmed by the joy of having all of her siblings back together for the first time since the pandemic. At the reception, guests ate smoked hams from pigs raised by the groom’s parents, corn grown by Ms. Driedger’s mother and a cake decorated by Mr. Driedger’s mom.

The next day, the couple left on their honeymoon, a road trip down the Atlantic seaboard to the Florida Keys.

“Thinking of Josiah and I, it was just the best day of my life,” Ms. Driedger says. “But then, thinking of what that caused, it’s like the worst day of my life.”

Precisely what it caused did not begin to come clear until five days after the ceremony. By then, the newlyweds were in New Jersey. Most other wedding guests had returned home – large numbers of them to Ontario. Unbeknownst to them, Ms. Driedger’s sister from Thailand was in hospital in New Brunswick.

On Oct. 23, a shocking message appeared on the family group chat: “Lea tested positive for measles.”

The illness sets in

Lea Knelsen was partway through an 18,000-kilometre trip around the planet when she began to feel unwell. She left her home in Narathiwat, Thailand, on Oct. 15, with an itinerary that included stops in Bangkok, Manila, Vancouver and Toronto before arriving in Fredericton.

The trip did not go as planned. A cancelled flight left her with an unexpected overnight in the Philippines. By the time she arrived in Vancouver, she had a headache. Her son, who was travelling with her, spotted a rash on her neck. She felt tired.

It was, she figured, nothing more than the punishment of a lengthy trip.

“It’s just been crazy,” she recalls thinking to herself. “I’m going to get over this. It’s nothing serious.”

Ms. Knelsen arrived the evening before the wedding. That night, she had a fever. The next day, her throat was painful. She popped an Advil. After circling the globe for the ceremony, she was not going to miss it.

“I thought, ‘I just need to get through the day.’”

It was only after the festivities that she decided the time had come to seek help. The nearest care facility was the local Upper River Valley Hospital, the newest hospital in New Brunswick, built in the wake of SARS and deliberately equipped to deal with contagious respiratory diseases.

The doctor who saw Ms. Knelsen grasped for answers.

Maybe the rash was scarlet fever? Maybe her sore throat meant she had strep, which her son had recently gone through? A strep swab tested negative, but the doctor said he didn’t trust the rapid test. He sent her home with antibiotics.

But her throat continued to worsen. She could not eat. She could not keep down the antibiotic pills.

“It just went from bad to worse,” she says.

“I was still conscious, and I was still there. But I just felt like everything was so far away.”

By Sunday, she had no choice but to return to the hospital. Breathing had grown difficult, and a CT scan revealed pneumonia in one of her lungs. Her throat was so sore that it became impossible to swallow. Repeated bouts of diarrhea kept her from sleep, but she was too weak to walk to the bathroom. Doctors said her liver had stopped functioning properly. They told her she was going into septic shock, blood poisoning that can kill in as little as 12 hours.

“I didn’t know if I would be able to come out of it, honestly.”

Could it have been prevented?

Not once did the thought of measles cross Ms. Knelsen’s mind. She had not taken notice of the outbreak spreading around her home in southern Thailand. Besides, she had never feared measles as anything more consequential than the chickenpox. Ms. Knelsen was born in Quebec, and moved to Asia in 2014, where she has been teaching English and studying Thai.

She had not been vaccinated. Nor had her children.

“I always thought, ‘Oh, most of those diseases are not really around any more,’” she says. “It never really crossed my mind that I would get something that was actually serious.”

The hospital drew a series of blood samples, but confirmation that Ms. Knelsen had the measles did not come until Wednesday. Five days had passed since the wedding. The guests had dispersed and, with them, the measles.

The provincial chief medical officer of health issued its first public notice the following day, warning only about possible exposure at the hospital and on a flight from Toronto. (Details about the flight’s connection in Manila were not released until nearly a week later).

No one could have known at the time, but the sparks of an outbreak had been fanned across the continent, first to Ontario – where the first case was reported 12 days after the wedding.

Ms. Knelsen was not alone in failing to consider the possibility of the measles. In New Brunswick, the medical profession, too, did not immediately grasp what had grabbed such fevered hold of her body.

Had doctors immediately diagnosed Ms. Knelsen with the measles, they stood a chance at halting the outbreak before it could begin. Rapid detection could have alerted wedding guests to their exposure, allowing them to isolate themselves.

“If they had diagnosed it earlier, maybe then they would have put in precautions so it doesn’t spread,” said Zahid Butt, an epidemiologist who is the Canada Research Chair in Interdisciplinary Research for Pandemic Preparedness at the University of Waterloo.

But, he cautions, it takes time to identify unusual diseases. And measles, until recently, was so infrequently seen that doctors could go an entire career without encountering a case.

It’s worth considering what constitutes an error in medicine before rushing to assign blame, said Hanif Chatur, an emergency room physician at Upper River Valley Hospital who was not among the physicians who saw Ms. Knelsen. He acknowledges that “there was a window of opportunity” to slow the initial measles spread. Still, he’s not sure it’s reasonable to expect a physician to instantly diagnose a disease that had been eliminated in Canada. A rash, after all, is a symptom of many other more typical ailments.

“Nine times out of ten, it’s going to be what’s common,” says Dr. Chatur. It is, he said, not unrealistic to confirm a rare diagnosis on the second or third visit.

At first, he says, “when you hear hoofbeats, you think horses. You don’t think zebras. Measles is still a zebra.”

It is, however, a zebra that moves with extraordinary speed.

The Canadian government believes the vast majority of cases in this country were related to that very first case in New Brunswick.

“The current outbreak in Canada in under-vaccinated communities was initiated by an internationally imported case from outside of North America who attended an event in New Brunswick that had attendees from a number of Canadian provinces,” Anna Maddison, a spokesperson for the Public Health Agency of Canada, said in a statement this summer in response to questions from The Globe.

Many of the initial people to get sick were in Ms. Knelsen’s family. All of her children came down with the measles. Her brother, too, fell grievously ill.

“Every single person that was at the wedding that did not have the vaccine got the measles,” says Marc Villeneuve, her father. “Most of them came from Ontario. That’s why Ontario was so bad, because it went from here to there. And then they have lots of relatives in Alberta.”

The Villeneuve family are devoted Christians, but not Mennonite. However, some of their spouses have a Mennonite background – Josiah among them – connecting the virus with that community and the places it has set roots across North America.

From there, infections continued to spread.

Outside of their community, the Villeneuves hadn’t talked publicly about what their family had been through. They were, however, open when asked to revisit that time. And looking back now, the family can’t help but think of the time that elapsed between Ms. Knelsen’s first hospital visit – hours after the wedding – and the eventual measles diagnosis the following week.

“We wish we would have known sooner so we could have told all those people: ‘Stay put, don’t go anywhere. Stay home,’” says Sylvie Villeneuve, Ms. Knelsen’s mother. “But a week later – well, people have gone everywhere.”

‘We have to move faster’

Nearly a year after the wedding, the measles continues to find new hosts to infect. This summer, authorities in Alberta took the dramatic step of recommending that parents keep babies and others at high medical risk away from the Calgary Stampede. The number of confirmed cases in that province alone has eclipsed the total across the entire U.S., where small outbreaks have more recently surfaced in Wisconsin and Arizona.

None can match Canada. Of more than 10,000 people sick with measles throughout the Americas this year, nearly half were in this country. In late August, new cases continued to emerge in six provinces: Alberta, British Columbia, Manitoba, Nova Scotia, Ontario and, once again, New Brunswick.

As the virus loops back to the place it first began to spread through this hemisphere, at least one question remains unanswered.

Where did Ms. Knelsen contract the measles?

She has a suspicion.

In the weeks before her trip to Canada, her son became ill, a case of strep throat followed by an unknown ailment that left him with blood in his urine.

Trying to understand what was wrong, she bounced in and out of hospitals and clinics around Narathiwat, where the family have lived since 2019.

At one hospital, staff ushered Ms. Knelsen and her son out of a waiting room without giving much in the way of explanation. “At the time, I didn’t really think about it that much,” she says. But she remembers another child was in the room at the time. “Were they suspicious that small child had measles, and they knew how contagious it was? That’s the only thing I could put my finger on.”

The administration of that medical facility, the Rajnara Hospital, disputed that possibility, saying staff at the time were posted at the hospital entrance to divert measles patients to a specialized ward.

“There was a measles outbreak,” says Abdul Kadir Yusof, the director of Rajnara Hospital. The hospital is situated 15 kilometres from the Pattanasat Witaya School, but in a region where measles is endemic and an outbreak was spreading widely, it’s still uncertain exactly where it first took hold. With a virus capable of lingering in the air for hours, Ms. Knelsen “could have easily gotten the measles anywhere.”

Such is the daily risk in a place where measles is endemic. Such is the future that attends countries that lose their elimination status.

From one perspective, exactly how Ms. Knelsen got the measles – and how it spread from New Brunswick across the continent – holds only limited meaning. The declining rates of measles vaccination in Canada meant it was only a matter of time before a viral spark caused ignition.

“It doesn’t matter where it came from,” says Ms. McGovern, the Yale lecturer who has tracked the outbreak. “This is a problem we’re going to continue to have.”

There is, though, good reason to trace the precise origins of this outbreak. It offers a chance not only to understand the common elements of humanity that have provided space for the virus to replicate, but also the ability to learn from others seeking to keep it under control.

Thailand once believed it could eliminate measles by 2023. Public health officials there say it may be possible to rid most of the country of the virus by 2026 – but not in the deep south where, in recent years, as few as 30 per cent of children were vaccinated in some areas.

“We usually say that measles move fast, we have to move faster,” says Darin Areechokchai, a senior official with the bureau of vector borne diseases, in the Thai Department of Disease Control.

In Canada, that may mean new strategies are needed. Public health has tended to encourage vaccinations for children. More needs to be done to deliver that message to those leaving the country, said Prof. Butt, the epidemiologist at the University of Waterloo. “You should basically say that if you are travelling to countries where there are ongoing measles outbreaks, you should get vaccinated,” he said.

That might offer an additional defence against new cases from abroad.

For now and into the foreseeable future, though, the virus is winning, exploiting cracks in human trust.

In the village of Tah-Loh Nang, the vaccine hesitancy local health workers confronts isn’t much different from what a doctor might hear in North America: Some believe measles really is nothing to fear. Others put their medical faith in traditional remedies, like casting spells. Memories of pandemic restrictions, meanwhile, have left such strong vaccine resentments that nine in 10 parents now refuse to allow their children to be vaccinated at school.

Some Thai parents remain spooked by stories of a man who, they heard, was unable to walk after getting the COVID vaccine. Doctors doubt the man’s ailment was related to the vaccine, but “when people tell that story, it creates fear,” Nutchaya Doloh, the local senior health officer.

Her response is personal outreach. On a recent afternoon, she hopped into a motorcycle sidecar and rode out past rubber plantations and durian groves, travelling in person to homes with young children.

“We go door to door to promote the vaccine,” she says.

It’s slow work.

But in Thailand – and now increasingly in North America, too – public health officials know the alternative is death.

Farooq Phiriyasart, an epidemiologist at Sungai Kolok Hospital in southern Thailand, likens the measles to a tsunami. When it hits, it affects everyone. But it is the vulnerable who are most at risk. “For the weakest, it’s very dangerous. Because they will die before the others,” he says.

The measles is a tsunami capable of reaching around the world. Higher ground is available in the form of a vaccine. But requiring people to accept it is bad policy, he believes, because it stirs resistance.

In southern Thailand, they have tried new ways to persuade people to voluntarily vaccinate. Older generations might have acted on the advice of a local imam. Today, young parents are more likely to put stock in what’s on their screens.

So public health authorities have gained access to school WhatsApp groups, where they posted messages likening vaccines to a passing storm that gives way to a brighter, healthier tomorrow. “Parents will do everything they can to protect their children – because their children are their most precious treasure,” one message read.

Rewards have also helped: When budgets exist, health workers have offered parents who vaccinate their children a kilogram of sugar or flour. “It’s very effective,” says Mr. Phiriyasart.

His advice to other countries now contemplating regular measles outbreaks? Be ready to respond when the measles does come – with stocks of vitamin A, which he credits with dramatically decreased death rates in last year’s outbreak – but also recognize that in an age of vaccine hesitancy, blanket solutions are unlikely to work.

“The solution should be tailor-made for each specific group of people.”

Maybe, he hopes, a younger generation will grow up more willing to vaccinate. Maybe, in a decade or two, the measles will be eliminated in southern Thailand, too. Maybe other countries will find ways to once again beat back the virus.

But, he says, the only way to get there will be to persuade people to trust again – in the vaccines, and in each other.

“We cannot force them.”