While most children who get COVID-19 have a mild or even asymptomatic illness, there are new reports that some children may have a complication that can be severe and dangerous. Called multisystem inflammatory syndrome in children (MIS-C) by the Centers for Disease Control and Prevention, it can lead to life-threatening problems with the heart and other organs in the body. Early reports compared it to Kawasaki disease, an inflammatory illness that can lead to heart problems. But while some cases look very much like Kawasaki’s, others have been different. Experts think that MIS-C is likely a reaction of the body to either a current or past COVID-19 infection — but there is much we don’t understand, including why some children with MIS-C have negative tests for COVID-19.