For decades, the United States has worked with the World Health Organization and others to quash polio, beating back to near extinction a merciless disease that once paralyzed hundreds of thousands of children each year.

That progress is in danger as the pandemic forces health-care providers to suspend door-to-door vaccination campaigns that have slashed the number of infections.

New cases have emerged in ­Niger and Chad, and fears have mounted about a resurgence in Afghanistan and Pakistan. Although the number of new polio infections has been small — 155 confirmed cases since January — even blips are worrisome, public health experts say, because small increases can trigger explosive outbreaks, especially in poor countries without the resources to combat the virus alone.

“The polio virus is being let free,” said Abdul Qadir, a health worker who has spent the past eight years delivering the vaccine in western Pakistan.

The polio program also risks becoming collateral damage in a political controversy that could hinder eradication efforts long after the pandemic subsides.

Last month, President Trump threatened to cut U.S. funding for polio and every other WHO program over the U.N. agency’s coronavirus response, which he called “China-centric.” In the weeks since, the president and his aides have worked behind the scenes to sideline the WHO. They have also sought to deflect criticism of the United States’ handling of the crisis by pointing to the WHO’s failure to demand more transparency and accountability from China in the earliest stages of the outbreak.

While many say the WHO legitimately warrants scrutiny, there is consensus in the public health community that stripping the agency of hundreds of millions of dollars earmarked to combat polio and other diseases could be devastating.

“Ultimately, the impact is not on the institution, it’s on health,” said Jen Kates, senior vice president and director of global health and HIV policy at the Kaiser Family Foundation.

Of the $893 million the United States sent in the 2018 and 2019 funding period, $237 million was an “assessed contribution” to the Geneva-based agency — a type of state membership due that may prove hard to cut without congressional approval.

At greater risk is the “voluntary contribution,” money provided to U.S. agencies for health efforts and then given to WHO programs. The largest share of this money goes to polio eradication, with large chunks to fight vaccine preventable disease, malaria, tuberculosis, HIV/AIDS and the provision of basic health care.

Pulling U.S. money from polio could undercut a system of disease surveillance that could be central to combating this pandemic and those is the future.

The cuts are “certainly not a good idea,” said Thomas J. Bollyky, director of the global health program at the Council on Foreign Relations. “Americans have a stake in the ability to control these viruses.”

When Hamid Jafari, a doctor and infectious disease specialist who now leads one of the WHO’s regional polio eradication programs, started his work in 1994, the disease paralyzed hundreds of thousands a year.

Over a nearly 30-year career at the Centers for Disease Control and Prevention — including 16 years seconded to the WHO — he and colleagues beat back the virus. “We made so much progress. We are 99 percent of the way,” he said.

But the past few years have been tough. Conflict has made some children hard to reach. Disinformation about vaccines has fueled skepticism and attacks on health workers.

The outbreak of the novel coronavirus made things worse.

Afghanistan, Pakistan and more than a dozen African countries have halted or postponed vaccine deliveries, citing pandemic-driven travel restrictions, overburdened health-care workers and an inability to administer the drugs at a safe distance.

In Congo, at least 86,905 children did not receive the oral polio vaccine during the first two months of the year, according to the United Nations Children’s Fund. The drop-off in doses could leave children particularly vulnerable as attention shifts to covid-19, the agency warned this past week.

Misinformation on social media has marred progress in the eradication fight, said Kevin Bekolo, a public health doctor in western Cameroon. Posts about vaccines containing the coronavirus have gone viral.

“This had already turned most of the population against the polio vaccination campaign,” Bekolo said, “even before it was postponed.”

Though it is hard to count cases during periods of lockdown, at least 155 cases have emerged worldwide since January, according to the Global Polio Eradication Initiative — a 182 percent jump from this time last year.

Sylvester Maleghemi, a Nigerian doctor who leads the WHO’s polio immunization team in South Sudan, said his job has transformed since the respiratory threat reached the East African country.

“It’s now 80 percent coronavirus, 20 percent polio,” he said.

His team didn’t want to spread the coronavirus, so after consulting with South Sudan’s health officials, it suspended the door-to-door vaccine program.

The nation hasn’t seen a case since 2014, but Maleghemi fears a resurgence.

“I don’t sleep at night sometimes,” he said. “I just ask myself: Where next might I have an outbreak?”

Public health professionals say suspending vaccination will have grave consequences because much of the developing world relies on an oral vaccine that contains live virus.

The oral droplets are roughly 10 times cheaper than the injectables used in the United States and Europe, which employ an inactivated virus and do not carry the same threat.

Vaccine-derived infections spread after children who have received the droplets use the restroom in areas without water filtration plants. A mutated strain of polio grows in the water supply and can infect those who drink from the tap before they have immunity. (Naturally occurring polio, known as wild polio, also spreads through feces-contaminated food and water.)

“As soon as you let off, you get cases,” said Andrew Noymer, an associate professor of population health at the University of California at Irvine.

Such was the case in the West African nation of Niger, which reported two new cases of polio in April.

Doctors blamed the stalled vaccine campaigns. Workers who administer the droplets are not allowed to travel because of the pandemic, said Anya Blanche, the WHO’s representative in Niger.

“The challenges are daunting,” she said

Cutting funding to the WHO adds another challenge.

The White House wants to divert money from the WHO to other organizations. The State Department is already advising employees to start finding new partners.

In a limited number of cases, that could work, experts said. But the agency’s expertise and deep networks in the world’s most hard-to-reach places will be tough to replace, especially for polio.

“There is no substitute for WHO in this instance,” said J. Stephen Morrison, director of the Global Health Policy Center at the Center for Strategic and International Studies.

Health experts are worried about explosive outbreaks, he said. “Having the U.S. throw this wrench into the works doesn’t help matters.”

For those on the front lines the hope is that Trump, having got the organization’s attention, will retreat from the funding threat.

In interviews, Secretary of State Mike Pompeo has suggested that the United States might continue to support WHO work on polio and other diseases.

“Let’s see if there’s a piece of this which we ought to continue to participate in,” Pompeo told “The Jack Heath Radio Show” on May 7, “because it’s doing good work on polio or whatever it may be,” he said.

“But then let’s make sure that when the next risk arises,” he continued, “when the next risk from a pandemic arises, we need an institution that’s going to deliver good outcomes for the American people.”

On the front line of the polio fight, health workers are waiting out lockdowns and worrying about the children at risk.

Qadir, the health worker in Pakistan, has worked through conflict and death threats — and will keep working. For now, he has been detailed to the coronavirus response.

As soon as it is safe, he will be back in the polio fight, he said. “The children we don’t reach may become paralyzed for life.”

George reported from Islamabad, Pakistan. Haq Nawaz Khan in Peshawar, Pakistan, contributed to this report.