Dozens more children are believed to have contracted the illness this year, but no one knows what’s causing the uptick.

A child being treated for enterovirus 68 in 2014. The virus has caused AFM in the past, but it’s still not clear what’s driving a seasonal uptick this year.

Federal health officials this week warned of a small surge in cases of a rare and mysterious polio-like condition affecting mainly children across the US.

The condition, called acute flaccid myelitis or AFM, has potentially sickened more than 250 people, the Centers for Disease Control and Prevention reported Tuesday. Of these cases, 90 have been confirmed — and there appear to be no geographic clusters in the outbreak so far.

Since the CDC began tracking AFM in 2014, they’ve detected between 22 and 149 cases every year. But this year’s AFM outbreak is notable because there’s already a spike in cases — and we still have more than a month to go. The current number is also significantly higher than the 33 cases confirmed last year, and the CDC hasn’t been able to figure out what’s driving the uptick.

AFM affects the nervous system — specifically, the area of the spinal cord called gray matter — and causes muscle weakness and paralysis in the arms and legs. Sometimes these symptoms are temporary, sometimes they persist, and doctors don’t understand why or who is most at risk.

The condition can be brought on by viruses, including enterovirus and West Nile, as well as environmental toxins. So people may become infected with these viruses or exposed to certain toxins, and then experience the polio-like symptoms of AFM. But the CDC hasn’t been able to isolate any viral link in this outbreak — and no one can figure out why.

“We’re not sure if the reason we’re not finding pathogens in all of these patients is because it’s cleared [from their systems]. Is it because it’s hiding? Is it because it’s something we haven’t tested for?” explained Dr. Nancy Messonnier, the director of CDC’s National Center for Immunization and Respiratory Diseases, in a press briefing.

That’s just one of the mysteries surrounding this year’s increase in cases. Here’s what we know and don’t know so far.

What we know

From August 2014 through November 2018, there were 414 confirmed cases of AFM across the US.

In 2014, 2016, and this year, more people were sick with AFM than in 2015 and 2017 — but the trends are difficult to interpret since the CDC only began tracking AFM in 2014, and doctors aren’t required to report cases to the agency.

The condition is rare, affecting fewer than one in a million people — even in years like this one, with more cases.

Most of the patients are children under the age of 18, with an average age of 4.

Common symptoms include sudden weakness and loss of muscle tone in arms or legs, as well as difficulty swallowing, facial or eyelid drooping, and slurred speech.

All the AFM cases that have been tested so far have tested negative for poliovirus.

The last major uptick in AFM cases happened in 2014 and coincided with an outbreak of enterovirus D68 (EV-D68). A respiratory illness, it spreads from person to person by coughing or sneezing, and typically manifests with cold-like symptoms — but can also lead to sudden paralysis and infections in the heart and brain.

One child died of AFM in 2017, and there have been no deaths reported in 2018.

Parents can protect their kids from serious illnesses like AFM by washing their hands, making sure vaccines are up to date, and using insect repellent to prevent mosquito bites.

What we don’t know