New York City officials also detected polio in wastewater last week.
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Health officials say we should all be worried about this news.
"Even one case of paralytic polio is a public health emergency," says Dr. Adam Ratner from NYU Langone Health. "It's conceivable that if you see one case that's it's one very unlucky person and there's not community spread, but it's much more likely there are hundreds and thousands of cases that gave us that one case of paralytic polio."
Severe impacts of polio include meningitis and paralysis.
To date, there is no cure for the virus.
"Even in places where polio has been under control, we are still at risk if we don't continue to vaccinate the population," Dr. Ratner says.
"What we're seeing now is evidence that we are still at risk, even though there hasn't been polio in this country for many years."
People who aren't vaccinated against polio are being urged to do so immediately.
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That's leaving many people scrambling to find out if they're protected.
Parents have been getting texts across the country from their grown kids, looking for their childhood immunization records.
If you can find your records, look for IPV or OPV, that's the oral polio vaccine.
If you can't find them, doctors say there's no harm in getting a new vaccine.
You can also request copies of your immunization forms.
Click here to access records in Pennsylvania, here for New Jersey and here for Delaware.