The disease has been found in samples of local wastewater, in a county nearby, and in sewage from New York City.
The region, located 48 kilometres (30 miles) north of Manhattan, has a polio immunisation rate that is significantly lower than the national average.
Compared to the statewide average of 79 percent, only 60 percent of two-year-olds have gotten a vaccination.
In the US, polio last occurred naturally in instances that were documented in 1979.
The incident that was identified in July included a younger man who had not had vaccines and was paralysed
Orthodox Jew and communicator Shoshana Bernstein is informing community members about the value of vaccinations.