Here's a question an everyday user might ask about polio: "I thought polio was wiped out — so why are we still hearing about it?

Polio has not actually been fully eradicated yet, which is why it continues to appear in the news. The disease has been eliminated from most of the world, but it persists in a small number of countries and occasionally resurfaces in places that had previously declared themselves polio-free. As of recent years, wild poliovirus transmission has been confirmed in Afghanistan and Pakistan, the last two countries where the virus has never been fully stopped. These regions face unique challenges including difficult terrain, political instability, armed conflict, and in some areas deep-seated mistrust of vaccination campaigns, all of which make it extremely hard to reach every child with the oral polio vaccine. There is also a second, less intuitive reason polio keeps making headlines, and it involves something called vaccine-derived poliovirus. The oral polio vaccine, which is widely used in lower-income countries because it is cheap and easy to administer, contains a weakened but live form of the virus. In communities with very low vaccination rates, this weakened virus can spread from person to person and, over time, mutate into a form that can cause paralysis just like wild poliovirus. This is not a reason to avoid vaccination, because the risk is extremely low and the vaccine protects far more people than it harms, but it does mean that outbreaks of vaccine-derived polio have occurred in parts of Africa, Asia, and even in countries like the United States and the United Kingdom, where cases were detected in wastewater and in at least one unvaccinated individual in New York in 2022. That New York case was a significant wake-up call for many Americans who assumed polio was a problem of the distant past. The patient developed paralysis, the first such case in the US in nearly a decade, and surveillance of sewage systems in the New York area revealed the virus had been circulating silently in the community for some time. This is possible because the vast majority of people infected with poliovirus never develop symptoms, or experience only mild flu-like illness, while a small percentage suffer paralytic disease. The virus can spread through contaminated water and fecal matter, meaning it can move through a community largely undetected until someone becomes paralyzed. The New York situation highlighted that pockets of unvaccinated people, even in wealthy countries with strong health infrastructure, can create vulnerabilities. The global effort to eradicate polio has been one of the most ambitious public health campaigns in history, led by the Global Polio Eradication Initiative, which involves the World Health Organization, UNICEF, the US Centers for Disease Control, Rotary International, and the Bill and Melinda Gates Foundation. Since the initiative launched in 1988, the number of wild poliovirus cases has dropped by more than 99 percent, from an estimated 350,000 cases per year to just a few hundred annually. That is an extraordinary achievement, but eradication requires getting to zero, and zero has proven elusive. The closer you get to eliminating a disease, the harder the remaining cases become to reach, because they tend to be in the most isolated and difficult circumstances. The reason this matters so much is that polio, unlike some other diseases, cannot be managed indefinitely through treatment. There is no cure for paralysis once it occurs. Vaccination is the only tool available, and it only works if coverage is high enough to prevent the virus from finding unvaccinated hosts. If global vaccination efforts were to slow down or stop before eradication is complete, the virus could rebound rapidly, potentially spreading to countries that have long since stopped routine vaccination and therefore have growing populations with no immunity. So while polio may feel like a relic of the 1950s to many people in wealthy countries, it remains an active and serious concern for global health authorities, and the occasional cases that appear in places like New York or London serve as reminders that no country is entirely insulated from a disease that still exists anywhere in the world.