At the beginning of this year, there was a thrum of excitement among global health experts: Eradication of polio, a centuries-old foe that has paralyzed legions of children around the globe, seemed tantalizingly close.

Pakistan, one of only two countries where wild poliovirus still circulates, had not recorded cases in more than a year. Afghanistan had reported only four.

But eradication is an uncompromising goal. The virus must disappear from every part of the world and stay gone, regardless of wars, political disinterest, funding gaps or conspiracy theories. New signs of the virus in a single country can derail the effort.

In polio’s case, there were several ominous setbacks.

Malawi in February announced its first case in 30 years, a 3-year-old girl who became paralyzed following infection with a virus that appeared to be from Pakistan. Pakistan itself went on to report 14 cases, eight of them in a single month this spring.

In March, Israel reported its first case since 1988. Then, in June, British authorities declared an “incident of national concern” when they discovered the virus in sewage. By the time New York City detected the virus in wastewater last week, polio eradication seemed as elusive as ever.

“It’s a poignant and stark reminder that polio-free countries are not really polio-risk free,” said Dr. Ananda Bandyopadhyay, deputy director for polio at the Bill & Melinda Gates Foundation, the largest supporter of polio eradication efforts.

The virus is always “a plane ride away,” he added.

Polio is a highly contagious and sometimes deadly enemy, capable of ravaging the nervous system and causing paralysis within hours. Those who recover could relapse and become seriously ill years later.

The virus multiplies in the intestine for weeks and could spread through feces or contaminated food or water — for example, when an infected child uses the toilet, neglects washing hands and then touches food.

For decades the virus terrorized families, causing paralysis among more than 15,000 American children each year and hundreds of thousands more worldwide. Its retreat is a triumph of vaccination. After the first vaccine arrived in 1955, the number of cases dropped precipitously, and by 1979 the United States was declared polio-free.

Although the United States and Britain have high immunization rates, they also have pockets of low immunity that allow the virus to flourish. In those communities, all unvaccinated people — not just children — are at risk. If polio continues to spread in the United States for a year, the country may lose its polio-free status under W.H.O. guidelines.

The Covid-19 pandemic left many other countries vulnerable to a resurgence of polio: It disrupted vaccination drives for months and diverted staff and resources away from prevention programs, resulting in the worst backslide in immunization rates in 30 years.

“The moment you take your eye off the ball, you know that the virus will simply reappear,” said Aidan O’Leary, director for polio eradication at the World Health Organization. “We have to literally face down every single chain of transmission that we can identify.”

Aid organizations first aspired to eradicate polio in 1988 and poured billions of dollars into the Global Polio Eradication Initiative, a consortium of six partners, including the Gates Foundation, the W.H.O. and the Centers for Disease Control and Prevention.

Despite the recent cases, the progress is unmistakable: Global cases of polio have fallen by 99 percent — from 350,000 cases of paralysis in 1988 to about 240 so far this year.

That success “is both a miraculous thing and a thing that’s taken way, way longer than people expected,” said Bill Gates, who has taken a pointed interest in polio, in an interview in February. “Eradications are super hard, and they rarely should be undertaken.”

Ending polio has been particularly challenging.

There are three strains of the wild poliovirus. Type 2 was declared eradicated in 2015, and Type 3 in 2019. Only Type 1 poliovirus remains at large, and only in Pakistan and Afghanistan.

Until recently, there was good reason to be optimistic about Type 1’s demise. India and Nigeria were both considered impossible targets for polio elimination, but both achieved that goal.

“There were so many people who kept telling us you will never succeed in India,” said Dr. Hamid Jafari, W.H.O.’s director of polio eradication for the eastern Mediterranean region.

Afghanistan and Pakistan have proven more difficult because of their nomadic populations, rough terrain and the baseless notion that the vaccine is a Western tool for sterilizing the population, Dr. Jafari said.

In Afghanistan, polio thrived in areas where immunization bans were imposed by the Taliban. In late March, the Taliban allowed vaccinations to resume, but the doses are administered in door-to-door campaigns, often by female health care workers. Some have been assaulted and killed.

Only one human viral disease, smallpox, has ever been eradicated. For all its deadliness, smallpox was relatively simple to dispatch because every infection resulted in dramatic, unmistakable symptoms.

Polio is much more sly: It can spread silently, causing mild flulike symptoms or none at all, and yet the disease paralyzes one of every 200 infected children. Even one case of paralysis is a signal that there may be hundreds or even thousands of undetected infections.

“Paralysis is the tip of the iceberg,” said Dr. Walter Orenstein, associate director of the Emory Vaccine Center and a former director of the United States’ Immunization Program.

But in some countries, polio has become such a dim and distant threat that health officials have stopped looking for it. While Britain and Israel monitor sewage for the virus — ideal because polio spreads through fecal matter — many others, including those in the United States, have ceased active surveillance.

“There’s no doubt that there are places where it needs to be reinforced,” said Dr. Matshidiso Moeti, W.H.O.’s regional director for Africa.

The single case imported into Malawi from Pakistan resulted in mass immunizations of nearly 28 million children in Malawi and its neighbors. But health care workers had become unaccustomed to door-to-door campaigns.

In the Chikwawa district in southern Malawi in March, Charles Bizimaki woke at 5 a.m., took the lunch his wife had packed for him and walked several kilometers to a nearby village. Mr. Bizimaki has been the vaccine manager for six villages since 2007.

But he had not conducted a door-to-door vaccination campaign since a tetanus outbreak in 2013 and had never led one for polio.

The campaign was physically exhausting and frustrating because it sometimes took multiple visits before he could find a child at home. “It was not an easy job,” Mr. Bizimaki said. It took him six days to vaccinate every child under 5 in the nearby villages.

Immunization for polio can be done in one of two ways. The injected vaccine used in the United States and most rich countries contains killed virus, is powerfully protective against illness but doesn’t prevent the vaccinated from spreading the virus to others.

Mass vaccination campaigns rely on the oral polio vaccine, which delivers weakened virus in just a few drops on the tongue. The oral vaccine is inexpensive, easy to administer and can prevent infected people from spreading the virus to others, a method better suited to extinguishing outbreaks.

But it has one paradoxical flaw: Vaccinated children can shed the weakened virus in feces, and from there it can sometimes find its way back into people, occasionally setting off a chain of infections in communities with low immunization rates.

If the weakened virus circulates for long enough, it can slowly mutate back into a more virulent form that can cause paralysis.

Even as wild poliovirus has been on the decline, so-called vaccine-derived polio has been on the upswing. Cases tripled between 2018 and 2019, and again between 2019 and 2020. Between January 2020 and April 2022, 33 countries reported a total of nearly 1,900 cases of paralysis from vaccine-derived polio.

The samples found in London sewage, in Israel and in New York are all vaccine-derived virus. They carry the same genetic fingerprint, suggesting that the virus may have been circulating undetected for about a year somewhere in the world.

Eradicating polio would require wiping out the vaccine-derived type, not just the few remaining hot spots of wild virus. “We definitely need to stop all polio transmission, whether wild poliovirus or whether circulating vaccine-derived poliovirus,” said John Vertefeuille, who heads polio eradication at the C.D.C.

Vaccine-derived polio has become more prevalent because the oral vaccine in use now protects against only Types 1 and 3 of the virus. In 2016, buoyed by the seeming eradication of Type 2 virus, the W.H.O. withdrew it from the oral vaccine. That move left the world increasingly vulnerable to outbreaks of residual Type 2 virus.

At the same time, global health organizations shifted away from maintaining nimble teams that can swiftly stamp out outbreaks to strengthening health care systems overall. Regions that struggle to contain polio tend to have other public health problems, such as poor nutrition, access to safe drinking water and other infectious disease outbreaks.

But the response to an outbreak of polio — or to other infectious diseases like Covid-19 or monkeypox — requires dedicated teams and programs, said Kimberly M. Thompson, a health care economist whose work focuses on polio eradication.

The W.H.O. has not delivered on that goal for decades, “but there is no accountability for performance,” Dr. Thompson said. Likewise, countries that receive funding for polio are rarely held responsible for diverting the money to other programs, she added.

As a result of the dismantling of outbreak teams, the response to vaccine-derived polio has often been sluggish and inefficient.

“The speed and the quality of the responses will have to go up in order for us to stop these outbreaks,” Dr. Vertefeuille said.

In November 2019, the W.H.O. granted an emergency use authorization for a novel oral vaccine that is specific to the Type 2 virus. The vaccine, which took a decade to develop, is more genetically stable than the widely used oral vaccine and less likely to revert to a form that can cause paralysis.

The eventual goal for polio eradication is to immunize children in every country with the injected vaccine used in the United States, said Jalaa’ Abdelwahab, director of vaccine programs at Gavi, which helps increase immunizations in poor countries. Supplies of oral vaccine would be stockpiled only to respond to unexpected outbreaks, Mr. Abdelwahab said.

The recent cases have forced a reassessment of the strategies being used to detect and contain polio. The C.D.C. is planning to introduce wastewater surveillance at strategic sites in the country, according to a statement from the agency.

Pakistan has among the largest wastewater surveillance systems for polio, but vaccine hesitancy is rampant. One team of scientists, led by Dr. Jai Das at Aga Khan University in Karachi, has found that offering communities an incentive — installing water pumps, for example — if they raise vaccination rates may be more effective than unconditional cash prizes for individuals.

Eradicating polio by 2026, the current goal, will require innovative strategies, patience and persistence — and an estimated $4.8 billion.

“That last mile, those last cases, are always the hardest,” said Simon Bland, chief executive officer of the Global Institute for Disease Elimination.