The move, announced by the U.S. Centers for Disease Control and Prevention on Wednesday, comes after a 20-year-old unvaccinated man in Rockland County, New York, was paralyzed by the virus in July—the first case reported in the U.S. in decades.

Other areas with low rates of polio vaccination or ties to at-risk New York communities linked to the recent case of paralytic polio will also participate in the wastewater testing program. No additional locations were specifically identified. The aim is to launch targeted vaccination campaigns if the potentially deadly virus is detected in sewage, the CDC said in a statement.

“It would not be surprising if poliovirus is detected by testing wastewater,” the federal health agency said. That’s because the virus is shed in the stools of those who have it, the majority of whom have no symptoms or symptoms that could be mistaken for the flu.

For many, polio conjures up mental images from history books—like former President Franklin D. Roosevelt, who was largely confined to a wheelchair by the virus. Since the July case, no additional ones have been reported in the U.S. But polio has “repeatedly” been detected by the CDC in wastewater from New York’s Rockland, Orange, and Sullivan counties, in addition to New York City and Nassau County, since, according to state health officials. 

Of the nearly 90 positive samples collected as of late October, more than 90% were genetically linked to the young man paralyzed this summer, according to the state health department. Experts caution that his case is likely only the tip of the iceberg, and that spread is undoubtedly occurring among others who are unvaccinated, most of whom are likely asymptomatic.

About a quarter of those who contract polio will experience mild flu-like symptoms, and only one in a few hundred will experience paralysis, Dr. Howard Forman, a professor of radiology, biomedical imaging, and public health at Yale School of Medicine, previously told Fortune.

With polio, the risk is almost exclusively to those who aren’t vaccinated. U.S. health officials currently recommend three doses of vaccine at 2, 4, and 6 months of age, with a booster between 4 and 6 years of age.

As of last year, 92.5% of U.S. 2-year-olds had been vaccinated for polio, virtually eliminating their risk of severe disease. But rates of childhood vaccinations have plummeted during the pandemic to the lowest levels seen in 30 years, the World Health Organization and UNICEF said in July. In 2021, nearly 7 million fewer children received the third dose of polio vaccine when compared to pre-pandemic 2019, the organizations said. A third jab elevates protection from 90% to 99%-100%, according to Harvard Health.

In 1994, the Pan American Health Organization declared polio as eradicated from the Americas. Since 2000, the U.S. has vaccinated children against polio with inactivated virus. But some other countries vaccinate with live attenuated virus that, on rare occasion, can mutate and spread via vaccinated people, who excrete it through their feces.

The virus is spread when someone comes in contact with fecal matter—even particles of it so small that they’re invisible to the naked eye—and then touches their mouth. It can be transmitted via food or water that is handled in unsanitary conditions. More rarely, droplets from a sneeze or cough of someone who has polio can infect someone else, according to the U.S. Centers for Disease Control and Prevention.