Of all the health concerns that were eradicated, Polio could be the worst.

Polio has been detected in New York City, the state Health Department said Friday, a jarring discovery based on wastewater samples that arrives three weeks after a case emerged in Rockland County.

Before the Rockland County case, the U.S. had gone about a decade without logging any polio cases. But the reported arrival of the paralyzing virus in New York City suggested community spread.

“For every one case of paralytic polio identified, hundreds more may be undetected,” Dr. Mary Bassett, the state health commissioner, said in a statement.

“The detection of poliovirus in wastewater samples in New York City is alarming, but not surprising.”

Dr. Ashwin Vasan, the city health commissioner, urged New Yorkers to respond to the news by getting jabbed with the long-administered polio vaccine.

“The risk to New Yorkers is real but the defense is so simple — get vaccinated against polio,” Vasan said in a statement. “Polio is entirely preventable and its reappearance should be a call to action for all of us.”

Ultimately, New York health officials will use wastewater monitoring to tell them quickly whether they have a bigger problem, essentially allowing them to test thousands of people at once for polio infection rather than individually, David Larsen, an epidemiologist and Syracuse University professor who directs the state’s wastewater surveillance network, said in an email.

Wastewater testing for polio has been a staple in developing nations for decades, but at least a few countries where cases are rare and vaccination rates are high do it, too.

The U.K. began monitoring wastewater in 2016 for polio and several other viruses that occur in the gastrointestinal tract, a spokesperson for the British health security agency said via email. (It has since added the virus that causes COVID-19 to the list.)

Israel has monitored sewage for polio since 1989. In 2013, health officials were able to detect an outbreak of wild polio just from sampling and launch a vaccine campaign in response without ever experiencing a case of paralysis. This year, though, a young child in the Jerusalem area came down with paralytic polio. Public health authorities there found additional infections through sewage tests.

Some U.S. public health officials have been skeptical of the value of such testing here.

“I’ve always been unenthusiastic about doing it for polio in the U.S. and a big supporter of doing it elsewhere, where there are deficiencies in other surveillance systems,” said Mark Pallansch, who retired in 2021 after spending much of his career working on polio eradication efforts for the Centers for Disease Control and Prevention.

COVID-19 has triggered a blast of interest in wastewater surveillance, prompting cities, states and colleges to launch programs and opening a floodgate of funding for them.

The CDC sent federal money to health departments in over 40 jurisdictions to support such tracking efforts, working with them to collect data that’s published on the agency’s National Wastewater Surveillance System website. A spokesperson said in an email that the agency was working to expand the platform to include data on other pathogens, from foodborne infections like salmonella to influenza, but not polio. Testing nationally for polio would be labor and resource intensive, requiring increases in public health laboratory capacity, the spokesperson said.

One asset of wastewater monitoring is the ability to pivot quickly to test something new.

In November 2020, the Sewer Coronavirus Alert Network, based out of Stanford and Emory universities, started daily monitoring at California wastewater plants for the virus that causes COVID-19. It’s since added monitoring for other pathogens, including COVID-19 variants, the common respiratory virus RSV and, most recently, monkeypox. Such additions are relatively economical since the network can test for multiple pathogens from a single sample, said Marlene Wolfe, one of the two principal investigators and an assistant professor at the Rollins School of Public Health at Emory.

In adding more tests, Wolfe said, the question is always whether monitoring a disease this way is likely to surface anything of enough concern to drive public health decisions.

Many question whether the expansion of wastewater testing fueled by the pandemic will last. Maldonado, the American Academy of Pediatrics’ infectious diseases committee chair, said the recent polio case is another signal that more disease tracking is critical.

“Maybe this is a clarion call for us to really start building better surveillance networks,” she said.