The US declared victory over polio in 1979. Now, it’s here again.

For the first time in almost a decade, a case of polio was confirmed in the United States. Health officials in New York’s Rockland County discovered the case last month in an unvaccinated 20-year-old, decades after polio was eliminated from the US in 1979.

With the country and public health system already struggling under the weight of Covid-19 and monkeypox, this news comes as an unpleasant surprise, and instantly raises questions. How did this happen? Who else is at risk? What does it mean that the Rockland case was a vaccine-derived strain, and what are the implications for...