For parents, one of the most reassuring aspects of the coronavirus has been that it largely spares children, only causing mild symptoms. But recent reports of a new life-threatening pediatric inflammatory syndrome are worrying many families. Children in Europe, Britain, and now the United States are being hospitalized for this mysterious illness, which can harm kids’ hearts and other organs, often requiring intensive care.

According to Gov. Andrew M. Cuomo, as of May 12, New York was investigating 102 cases of this syndrome and three deaths, and cases have been reported in 14 other states so far as well. Thankfully, this new syndrome is still quite rare and it is treatable, and most kids who develop it fully recover. Here’s what parents need to know and what they should do if they’re concerned about their children’s health.

No one knows for sure whether this new syndrome, which is now being called pediatric multisystem inflammatory syndrome, or P.M.I.S., is linked to the coronavirus, but many doctors think so. “I do think that this is related to Covid,” said Dr. Eva Cheung, M.D., a pediatric cardiologist and critical care specialist at NewYork-Presbyterian Morgan Stanley Children’s Hospital, which has treated more than 35 patients so far with the syndrome.

The vast majority of children who’ve fallen ill in New York have either tested positive for active Covid-19 infection or have had antibodies suggesting that they were exposed to the virus at some point. A few kids have tested negative for both active infection and antibodies, and it’s unclear what that means. It’s possible that these negative tests were inaccurate, Dr. Cheung said, because many coronavirus antibody tests are unreliable.