Health officials are expressing concerns as poliovirus has been detected in wastewater in Rockland County, New York.
The county first confirmed a case of polio in July, but the fact that the virus was detected in wastewater suggests they could be potentially hundreds of cases.
Wastewater samples also found the virus in Orange County, New York.
Earlier this week, Centers for Disease Control and Prevention (CDC) Director Rochelle Walensky said she was directing resources to the county, including expanding polio vaccinations.
Orange and Rockland County have the two lowest polio vaccination rates among toddlers in New York State, both at around 60%.
A community health official told CNN, “this is a silent killer, like carbon monoxide, and we don't know when it will hit us."
The CDC said that most people infected with poliovirus, the virus that causes polio, do not experience symptoms.
In severe cases of polio, it can cause permanent disability and death.
Nationally, 92% of children are fully vaccinated from polio by 24 months, the CDC said.