A purple mark on a toddler’s pinkie shows that two drops of polio vaccine have just been given on a busy street in Karachi, Pakistan. Across the country, thousands of health workers go door to door during vaccination drives, making sure children are protected against a virus that can still cause lifelong paralysis.
In 2025, wild polio was found in only two countries: Afghanistan and Pakistan. It paralyzed 52 children there. That is a stark contrast with 1988, when polio paralyzed about 350,000 children in 125 countries. A global effort to wipe out the disease has cut cases by more than 99% since then. The progress shows what vaccines can do, while the remaining cases show how hard it can be to reach every child.
Polio spreads when tiny traces of an infected person’s poop get onto hands, food or water and then into another person’s mouth. It spreads more easily where clean water and good sewer systems are lacking. The virus is also hard to spot: most people who catch it never feel sick, even though they may pass it on. About 1 in 200 infections worldwide attacks the nerves that control muscles, usually in the legs, and can leave a person paralyzed for life. There is no cure, but a few vaccine doses can protect someone for life.
The vaccine can be given as drops or as a shot. In Karachi, workers use the oral drops and mark vaccinated children so their doses are easy to track. In the United States, children usually get a shot at a doctor’s office. Both forms protect against polio, but vaccination must reach enough children to stop the virus from spreading. Of the three types of wild poliovirus that once circulated, only type 1 remains.
Conflict and distance make that work harder in Afghanistan and Pakistan. Some areas are too dangerous for vaccinators to enter, and workers have been attacked. In Afghanistan, house-to-house vaccination has not taken place since late 2024. Families are asked to bring children to central locations such as mosques, but children who cannot get there miss their chance.
In places where too few children are vaccinated, a separate risk can emerge: vaccine-derived polio. The oral vaccine contains a weakened virus that trains the body to fight infection. It can pass out in a child’s poop, but when nearly all children nearby are vaccinated, it has little chance to spread. Where fewer than 80% are vaccinated, it can move from child to child and change over time until it can cause paralysis. The problem is not that the vaccine is unsafe; it is that too few children are protected.
The children most likely to miss doses are often in poor families, remote villages or war zones. One study in Pakistan found that offering families other free health care alongside polio drops cut the share of children who had never been vaccinated nearly in half. That points to a practical way forward: bring vaccination closer to families and pair it with care they need. The virus has been pushed back across most of the world; reaching the children still left out is essential to keeping that progress going.