Why do people still get polio in some countries and not in others?

Polio is an infectious disease caused by a virus that lives in the human gut. It spreads when tiny amounts of infected poop reach hands, food, or water and then enter another person’s mouth. Most infected people feel completely well, which lets the virus spread unnoticed.

In about 1 out of every 200 infections globally, polio attacks the nerves that control muscles. When those nerves are damaged, the muscles may stop working. The resulting paralysis usually affects the legs and can last for the rest of a person’s life. There is no cure for polio.

Vaccination is the main protection. A few doses can protect someone for life, and high vaccination coverage prevents the virus from reaching unprotected children. This is why polio can travel into vaccinated places without paralyzing children there.

Wild polio, meaning the original virus, now remains in only two countries: Afghanistan and Pakistan. This is a dramatic change from 1988, when polio paralyzed about 350,000 children across 125 countries. Global vaccination has pushed the virus into a very small number of places.

In 2025, wild polio paralyzed 52 children in Afghanistan and Pakistan. Both countries have experienced decades of war. In some areas, vaccinators cannot travel safely, and some health workers have been attacked. These conditions make it difficult to reach every child with protective vaccine doses.

Afghanistan stopped house-to-house vaccination late in 2024. Families must instead take children to central locations, such as mosques, so children who cannot reach those sites may miss vaccination. Wild polio could spread further if gaps in vaccination remain, even though the virus is now geographically restricted.

Polio outbreaks continue where too few people have received preventive vaccines. The virus spreads silently because most infected people feel healthy. It then finds vulnerable children in places with low vaccination coverage, especially communities that are poor, remote, or affected by war.

Germany shows the difference. In October 2025, scientists found wild polio in Hamburg sewage, with genes matching virus from Afghanistan. Someone carrying the virus had probably traveled there, but no German child was paralyzed because most children were vaccinated. The United States generally uses polio vaccine shots at doctors’ offices, while Pakistan uses door-to-door drops.

The danger is not limited to countries where wild polio remains. Vaccine-derived outbreaks can occur when coverage falls below about 80 percent. Keeping vaccination high, reaching missed children, and countering misinformation help prevent imported or vaccine-derived virus from causing paralysis.

Vaccination works only when health workers can reach children. In Afghanistan and Pakistan, decades of war have made some areas unsafe for travel. Vaccinators may be unable to visit homes, and attacks on health workers create additional barriers. Children in these areas can remain unprotected even when vaccination campaigns are active elsewhere.

Afghanistan stopped house-to-house vaccination in late 2024. Families must bring children to central locations, including mosques. A child whose family cannot safely travel, or lives too far away, may miss the campaign. The article also identifies children in poor families, faraway villages, and war zones as among the hardest to reach.

These gaps matter because polio spreads quietly. An unvaccinated child may catch or pass on the virus without feeling sick. Reaching missed children is therefore essential. In Pakistan, offering other free health care alongside polio drops nearly halved the share of children who had never been vaccinated.

Too few vaccinated children can allow a weakened virus from the oral polio vaccine to keep circulating. The vaccine virus is designed to teach the body to fight polio without making a child sick. After vaccination, some of it leaves the body in poop. High coverage usually stops it from finding new hosts.

When fewer than 80% of children are vaccinated, the weakened virus can pass from child to child. Like a message in a telephone game, it changes slightly with each passage. Eventually, it can change back into a form capable of paralyzing children. This causes vaccine-derived polio.

The problem is low vaccination, not the basic purpose of the vaccine. Children who miss doses are often in poor families, remote villages, or war zones. Reaching them, addressing rumors, and maintaining strong coverage can stop the virus from circulating and protect communities from outbreaks.

In 1988, countries around the world joined a shared effort to wipe out polio. That year, polio paralyzed about 350,000 children in 125 countries. The central strategy was vaccination: giving children doses that build protection and reducing the number of people able to carry the virus.

Large campaigns brought vaccines directly to families. In Pakistan, health workers visit homes and give two oral drops, marking a child’s pinkie afterward. In the United States, most children receive the vaccine as a shot at a doctor’s office. These approaches differ, but both protect children from poliovirus. Over time, vaccination drove transmission down.

Polio has not disappeared completely, but cases have dropped by more than 99% globally since 1988. Three wild poliovirus types once existed; only type 1 remains. Continued progress depends on reaching missed children, protecting health workers, and maintaining high coverage so transmission cannot restart.

High vaccination coverage protects more than the people who received doses. When nearly every child is vaccinated, the virus has few opportunities to move from one person to another. This interrupts transmission and helps protect people who may encounter the virus, including those who are not fully protected.

The article gives a clear example from Germany. In October 2025, sewage testing in Hamburg found wild polio whose genes matched virus from Afghanistan. A person carrying the virus had probably traveled there, likely without feeling sick. Yet no child in Germany was paralyzed because most German children were vaccinated. Other European countries found vaccine-derived virus in sewage but also saw no paralysis.

This is herd immunity: community-wide protection created by high vaccination coverage. It does not mean the virus cannot enter. It means the virus struggles to spread far enough to cause disease. Maintaining coverage remains important because falling below about 80% can allow vaccine-derived virus to circulate.