Officials said the patient, who lives in Rockland County, had developed paralysis. The person developed symptoms a month ago and did not recently travel outside the country, county health officials said.

It appears the patient had a vaccine-derived strain of the virus, perhaps from someone who got live vaccine—available in other countries, but not the U.S.—and spread it, officials said.

The person is no longer deemed contagious, but investigators are trying to figure out how the infection occurred and whether other people may have been exposed to the virus.

Most Americans are vaccinated against polio, but unvaccinated people may be at risk, said Rockland County Health Commissioner Dr. Patricia Schnabel Ruppert. Health officials scheduled vaccination clinics nearby for Friday and Monday, and encouraged anyone who has not been vaccinated to get the shots.

“We want shots in the arms of those who need it,” she said at a Thursday press conference announcing the case.

Polio was once one of the nation’s most feared diseases, with annual outbreaks causing thousands of cases of paralysis—many of them in children.

Vaccines became available starting in 1955, and a national vaccination campaign cut the annual number of U.S. cases to less than 100 in the 1960s and fewer than 10 in the 1970s, according to the Centers for Disease Control and Prevention.

In 1979, polio was declared eliminated in the U.S., meaning there was no longer routine spread.

Rarely, travelers have brought polio infections into the U.S. The last such case was in 2013, when a 7-month-old who had recently moved to the U.S. from India was diagnosed in San Antonio, Texas, according the federal health officials. That child also had the type of polio found in the live form of vaccine used in other countries.

There are two types of polio vaccines. The U.S. and many other countries use shots made with an inactivated version of the virus. But some countries where polio has been more of a recent threat use a weakened live virus that is given to children as drops in the mouth. In rare instances, the weakened virus can mutate into a form capable of sparking new outbreaks.

U.S. children are still routinely vaccinated against polio with the inactivated vaccine. Federal officials recommend four doses: to be given at 2 months of age; 4 months; at 6 to 18 months; and at age 4 through 6 years. Some states require only three doses.

According to the CDC’s most recent childhood vaccination data, about 93% of 2-year-olds had received at least three doses of polio vaccine.

Polio spreads mostly from person to person or through contaminated water. It can infect a person’s spinal cord, causing paralysis and possibly permanent disability and death. The disease mostly affects children.

Polio is endemic in Afghanistan and Pakistan, although numerous countries in Africa, the Middle East and Asia have also reported cases in recent years.

Rockland County, in New York City’s northern suburbs, has been a center of vaccine resistance in recent years. A 2018-2019 measles outbreak there infected 312 people.

Last month, health officials in Britain warned parents to make sure children have been vaccinated because the polio virus had been found in London sewage samples. No cases of paralysis were reported.