At the beginning of the year, there was a lot of excitement among global health experts, who were hopeful that the world would soon be free of the disease.

Pakistan, one of the two countries where the wild version of the disease still circulates, has not had a case in more than a year. Afghanistan did not report many.

Eradication is not negotiable. The virus needs to disappear from every part of the world, no matter what. The effort can be derailed by new signs of the virus.

There were many setbacks in the case of the disease.

The first case of its kind in 30 years was announced in February, when a 3-year-old girl became paralyzed from the waist down after contracting a disease that looked like it came from Pakistan. Eight cases were reported by Pakistan in a single month.

Israel reported its first case in 25 years. The British authorities declared an "incident of national concern" when they discovered the virus in sewage. By last week, New York City had detected the virus in wastewater.

Dr. Ananda Bandyopadhyay said that it was a stark reminder that countries that are free of the disease are still at risk.

He said that the virus is always on a plane.

It's a disease that can cause paralysis within hours, and it's also a disease that can spread quickly. People who recover can get seriously ill years later.

When a child uses the toilet without washing their hands or touching food, they are at risk of contracting the virus, which can spread through feces or contaminated food.

ImageDr. Jai Das visited the neighborhood of Rheri Goth in Karachi, Pakistan, hoping to convince residents to get the polio vaccine.
Dr. Jai Das visited the neighborhood of Rheri Goth in Karachi, Pakistan, hoping to convince residents to get the polio vaccine.Credit...Khaula Jamil for The New York Times
Dr. Jai Das visited the neighborhood of Rheri Goth in Karachi, Pakistan, hoping to convince residents to get the polio vaccine.

Thousands of American children are paralyzed by the virus every year and hundreds of thousands more worldwide. It is a triumph of vaccine. The number of cases plummeted after the first vaccine arrived, and the United States was declared free of the disease in 1979.

The United States and Britain have low immunity that allows the virus to flourish. Unvaccinated people are at risk in those places. W.H.O. guidelines state that the country may lose its immunity to the disease if it continues to spread in the US.

The worst backslide in immunization rates in 30 years was caused by the Covid-19 outbreak, which disrupted vaccine drives and diverted staff and resources away from prevention programs.

The director of the World Health Organization said that if you take your eye off the ball, the virus will come back to haunt you. We have to look at every single chain of transmission.

The Gates Foundation, the W.H.O., and the Centers for Disease Control and Prevention are part of the Global Polio Eradication Initiative.

Despite the recent cases, the progress is clear: Global cases of the disease have fallen by 99 percent, from 350,000 cases in 1988 to about 120 this year.

Bill Gates said in an interview in February that the success of the vaccine was both a miracle and a long time in the making. Eradications are hard and should not be done.

It has been a challenge to finish the eradication of the disease.

There are three different strains of the vaccine. The eradication of type 2 and type 3 took place over the course of two years. Pakistan and Afghanistan are the only places in the world where you can still get a case of the disease.

There were good reasons to be optimistic about the demise of Type 1. India and Nigeria were both targets for the eradication of the disease.

There were so many people who told us that we would never succeed in India.

ImageMr. Bizimaki has been the vaccine manager for six villages in Malawi since 2007. Until recently, he had never needed to conduct a door-to-door vaccination campaign for polio.
Mr. Bizimaki has been the vaccine manager for six villages in Malawi since 2007. Until recently, he had never needed to conduct a door-to-door vaccination campaign for polio.Credit...Thoko Chikondi for The New York Times
Mr. Bizimaki has been the vaccine manager for six villages in Malawi since 2007. Until recently, he had never needed to conduct a door-to-door vaccination campaign for polio.

Afghanistan and Pakistan are more difficult due to their nomadic populations, rough terrain and the belief that the vaccine is a Western tool for sterilizing the population.

The Taliban banned immunization in Afghanistan, but that didn't stop the spread of the disease. The Taliban allowed vaccinations to resume in late March, but the doses are administered in door-to-door campaigns. The people have been killed.

There is only one human disease that has been eradicated. When it came to deadlines, smallpox was easy to dispatch because of the symptoms it caused.

The disease can be spread silently, causing mild flulike symptoms or no symptoms at all, and yet it can paralyze children. There could be hundreds or even thousands of undetected infections if one case of paralysis is any indication.

Dr. Walter Orenstein is an associate director of the Atlanta Vaccine Center and a former director of the U.S. immunization program.

Health officials in some countries have stopped looking for the disease because it is so far away. Many other countries, including the United States, have stopped active monitoring sewage for the virus because it is ideal for the disease.

W.H.O.'s regional director for Africa said there were places where it needed to be reinforced.

mass immunizations of nearly 28 million children resulted in the importation of a single case from Pakistan. Health care workers were not used to door to door campaigns.

Charles Bizimaki woke up at 5 a.m. and took the lunch his wife had packed for him and walked several kilometers to the village. The vaccine manager is Mr. Bizimaki.

He didn't conduct a door-to-door vaccine campaign since the tetanus outbreak and didn't lead a vaccine drive for the disease.

The campaign was tiring and frustrating because it took multiple visits before he could find a child to live with. Mr. Bizimaki said it was not easy. He had to give the vaccine to every child under the age of 5.

ImageMr. Bizimaki recorded vaccine data for a Malawian health registry during a springtime campaign.
Mr. Bizimaki recorded vaccine data for a Malawian health registry during a springtime campaign. Credit...Thoko Chikondi for The New York Times
Mr. Bizimaki recorded vaccine data for a Malawian health registry during a springtime campaign.

There are two ways in which the vaccine can be given for the disease. The injected vaccine used in the United States and most rich countries protects against illness but doesn't prevent the vaccine from spreading the virus to others

The oral polio vaccine can be given in a few drops on the tongue. The oral vaccine is inexpensive, easy to administer and can prevent people from spreading the virus to other people.

It has one flaw, that vaccinated children can shed the weakened virus in feces, and from there it can sometimes find its way back into people, setting off a chain of infections.

The weakened virus can slowly evolve into a more dangerous form that can cause paralysis if it continues to circulate.

The vaccine-derived form of the disease has been on the rise. There were more cases between the years of 2020 and 2019. 33 countries reported a total of 1,900 cases of paralysis from vaccine-derivedpolio between January 2020 and April 2022, according to the World Health Organization.

In London sewage is all vaccine-derived virus. It's possible that the virus may have been undetected for a year somewhere in the world.

The vaccine-derived type is the one that would need to be wiped out in order to eradicate the disease. The head of the C.D.C.'s anti-polio program said that all transmission of the disease needs to be stopped.

The oral vaccine protects against only types 1 and 3 of the virus, which has led to a rise in vaccine-derivedpolio. The W.H.O. withdrew the oral vaccine due to the eradication of the type 2 virus. The world was more vulnerable to an outbreak of residual Type 2 viruses.

At the same time, global health organizations shifted away from maintaining teams that can be quickly put out of business. Poor nutrition and access to safe drinking water are some of the public health problems that can be found in regions that struggle to contain the disease.

According to a health care economist, dedicated teams and programs are required to respond to an outbreak of a disease.

ImageDr. Das, of Aga Khan University in Karachi, has found that offering communities incentives like water pumps if they raise vaccination rates may be more effective than cash prizes for individuals.
Dr. Das, of Aga Khan University in Karachi, has found that offering communities incentives like water pumps if they raise vaccination rates may be more effective than cash prizes for individuals.Credit...Khaula Jamil for The New York Times
Dr. Das, of Aga Khan University in Karachi, has found that offering communities incentives like water pumps if they raise vaccination rates may be more effective than cash prizes for individuals.

There is no accountability for the W.H.O.'s performance. She said that countries that receive funding for the disease are not held responsible for redirecting the funds.

The response to vaccine-derived polio has been slow and inefficient because of the dismantlement of outbreak teams.

The quality of the responses will have to go up in order for the outbreak to be stopped.

The W.H.O. granted an emergency use authorization for a vaccine that protects against the type 2 virus. The vaccine is more stable than the oral vaccine and is less likely to cause paralysis.

Jalaa' Abdelwahab is the director of vaccine programs at Gavi which helps increase immunizations in poor countries. Mr. Abdelwahab said that the vaccine would be stored in a way that would be able to deal with an outbreak.

The recent cases have led to a rethinking of the strategies used to detect and contain the disease. According to a statement from the agency, the C.D.C. is going to introduce wastewater monitoring at strategic sites.

There is vaccine hesitancy in Pakistan but it is one of the largest. A team of scientists, led by Dr. Jai Das at Aga Khan University in Karachi, has found that giving communities an incentive to install water pumps may be more effective than giving people cash.

It will take innovative strategies, patience and persistence in order to eradicate the disease by the end of the century.

Simon Bland is the chief executive officer of the Global Institute for Disease Elimination.