The recent return of polio has hit like something of an epidemiological thunderclap. It was in 1979 that polio was officially declared eradicated in the U.S.—an early step in a multi-generational effort to wipe out the disease around the world. On July 21, however, the New York State Department of Health announced a case of polio in an unvaccinated man in Rockland County, and since then, circulating poliovirus has been found in wastewater there and in neighboring Orange County, as well as in New York City. In London, the virus was also found in wastewater in February, and in Jerusalem, a case of the disease turned up that same month.

The three cases, though seemingly isolated, point to a troubling trend—one that goes against more than three decades of progress in eradicating the disease. In 1988, polio was endemic in 125 countries and led to the death or paralysis of 350,000 people—mostly children—each year, according to the World Health Organization (WHO). View more