In the last two year more than 64 children across the United States have come to doctors with unexplained arm or leg weakness. Some also report difficulty swallowing or talking, double vision and show facial drooping. In Colorado alone, 10 children were hospitalized in August and September.

Doctors and the media have noted the symptoms' resemblance to the paralysis caused by polio, but no one knows what is causing this illness.

Polio-like paralysis does happen, explains Jake Nicol for National Geographic:

The CDC expects to see acute flaccid paralysis at a rate of about one in 100,000 globally for children under age 15 in countries where polio is not endemic. These cases are attributed to a range of causes, but often the source is never determined. In California, many health experts suspect that a non-polio enterovirus (polio itself is an enterovirus) may be the culprit of the polio-like illness. Enteroviruses are passed on through close contact with an infected person. While non-polio enteroviruses are extremely common, causing 10 to 15 million infections in the United States each year, primarily in infants, children, and teenagers, most people who are infected never get sick, or get cold-like symptoms at most, according to the CDC.

The illnesses' rarity and variability makes finding the cause difficult. Some children recover from the paralysis, and others don’t. Four of the children in Colorado tested positive for enterovirus 68, but the others did not. None of three Missouri patients tested positive for enterovirus 68, reports The New York Times.

So making the connection between that enterovirus—which has sickened more than 1,000 people in the U.S. since mid-August—and the polio-like symptoms has proved difficult so far. Enterovirus 68 is just one of the roughly 100 enteroviruses that circulate every year and has been around science 1962.

This odd cluster of symptoms in a short period of time put the Centers for Disease Control and Prevention (CDC) on alert. They have issued guidelines for physician to report any new cases. But at the same time, they emphasize that this illness appears to be very rare.

"At the moment, it looks like whatever the chances are of getting this syndrome are less than one in a million," said Mark A. Pallansch, the director of the division of viral diseases at the CDC., as reported in The New York Times.

“We don’t have a single clear hypothesis that’s the leading one at this point,” Pallansch told the Times. The CDC and physicians are working to rule out causes, and making sure that only truly similar cases are analyzed as part of the pattern —including look-a-like illnesses can muddy the picture.

If more cases surface, connections may turn up. Until then, the paralysis will remain mysterious.