In Pakistan, where wild polio still has not been eliminated, health workers go door to door vaccinating young children. ARIF ALI/AFP via Getty ImagesCurious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com.Why are people still getting polio in some countries and in not others? – Mia, age 9, Centennial, ColoradoOn a busy street in Karachi, Pakistan, a healthcare worker kneels down so she’s eye to eye with a toddler. She squeezes two drops of liquid onto the child’s tongue. Then she takes out a purple marker and colors the child’s pinkie fingernail so everyone can see that this toddler is now vaccinated against polio.During big vaccination drives, thousands of health workers like her knock on doors across Pakistan.Most kids in the United States will never experience anything like this, because they get the polio vaccine as a shot at the doctor’s office instead of drops in a door-to-door campaign.The two vaccines work a little differently, but both protect kids from the polio virus. The countries that still see polio outbreaks are places where not enough people have gotten the preventive vaccine.I’m an epidemiologist – a scientist who studies how diseases spread. My work focuses on diseases that vaccines can prevent, and on the false information that keeps some families from getting those vaccines.A sneaky virusPolio is caused by a virus that lives in the human gut. It spreads when tiny bits of poop from an infected person get onto hands, food or water, and then into someone else’s mouth. That’s why polio spreads fastest in places without clean water or good sewer systems.Polio is sneaky. Most people who catch it never feel sick at all, so they can pass it on without knowing.But in about 1 out of every 200 infections globally, the virus attacks the nerves that control muscles, usually in the legs. This can cause a person to be paralyzed for the rest of their life.There is no cure for polio. But there is a vaccine, and a few doses can protect you for life.How vaccines pushed polio backIn 1988, polio paralyzed about 350,000 children in 125 countries. That same year, countries around the world teamed up with one huge goal: Wipe out polio for good.Though polio has not yet been entirely eliminated, this effort has been a major success. Since then, polio cases have dropped by more than 99% globally. There used to be three types of wild polio virus. Now, only type 1 is left.Where wild polio still hidesWild polio, the original virus, now spreads in just two countries: Afghanistan and Pakistan. In 2025, wild polio paralyzed 52 children there.Both countries have lived through decades of war. In some areas, it’s too dangerous for vaccinators to travel, and some have even been attacked.In Afghanistan, vaccinators haven’t gone house to house since late 2024. Families must bring children to central spots, such as mosques, instead. But the kids who can’t get there miss their vaccines.When too few kids are vaccinatedSome countries have outbreaks of a different kind, called vaccine-derived polio.The oral polio vaccine, which is no longer used in many countries, including the U.S., contains a weakened polio virus – too weak to make a child sick, but strong enough to teach the body to fight polio.After a child swallows the drops of vaccine, some of the weakened virus leaves their body in their poop. If nearly every kid around is vaccinated, the virus has nowhere to go and fizzles out.But in areas where fewer than 80% of kids are vaccinated, it can pass from child to child. Like a message in a game of telephone, the virus changes a little each time it’s passed. Eventually, it can change back into a form that paralyzes children.That might sound like the vaccine is the problem. But in fact, the problem is that too few kids have received the vaccine.The children who go unvaccinated are usually the hardest to reach: kids in the poorest families, in faraway villages or in war zones. In one study in Pakistan, offering families other free healthcare along with polio drops cuts the share of never-vaccinated kids nearly in half.The polio that went nowhereIn October 2025, scientists testing sewage in Hamburg, Germany, found wild polio. Its genes matched virus found in Afghanistan, more than 3,000 miles (4,800 kilometers) away.This means that someone carrying the virus, probably without feeling sick, had traveled to Germany. But not a single child there was paralyzed because most German kids are vaccinated against polio.Other countries in Europe found vaccine-derived polio in their sewage in 2025, with the same happy ending.Polio can travel anywhere, but it can only hurt kids where they aren’t protected by the vaccine.When rumors stop vaccinesPolio also gets help spreading from rumors and misinformation. In 2003, false stories that the vaccine was harmful led parts of northern Nigeria to stop vaccinating for almost a year. This led to a small outbreak that spread from that area to other countries.Research shows that false news online travels “farther, faster, deeper and more broadly than the truth.”Unfortunately, misinformation could also contribute to a resurgence of polio in places where it has been eliminated – even the U.S. In Colorado, about 1 in 8 kindergartners haven’t had all their polio shots. And in 2022, vaccine-derived polio paralyzed an unvaccinated young man in New York.Though rumors can travel quickly, trust is built one conversation at a time, by neighbors, religious leaders and doctors who take parents’ questions and concerns seriously. A Pakistani health worker marks the pinkie of a child she has just vaccinated for polio. Pacific Press/LightRocket via Getty Images So close to the finish lineUnfortunately, the budget of the worldwide program to end polio decreased by 30% in 2026, mostly because wealthy countries cut their foreign aid. This cut is largely led by the U.S., which withdrew from the World Health Organization, or WHO, officially in January 2026. Other wealthy donor governments like Germany and the U.K. have also made cuts.The WHO has warned that these cuts mean that in some places, the fight against polio will not be able to continue.Only one human disease has ever been wiped off the planet for good: smallpox, in 1980. Polio could be the second. The world has never been this close.Somewhere today, a healthcare worker is coloring another tiny pinkie purple. Each one breaks a link in the chain that polio needs to survive.Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.Katrine L. Wallace does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.