Early in the pandemic, some children fought off COVID with few, if any, symptoms, only to go into organ failure a few weeks later. Most recovered after aggressive treatment, but their sudden illness, dubbed multisystem inflammatory syndrome in children (MIS-C), remained a mystery. Now, a team of scientists from UC San Francisco, Chan Zuckerberg Biohub San Francisco, St. Jude Children’s Research Hospital and Boston Children’s Hospital has discovered what led to many of these cases, with a study that has implications for other autoimmune diseases.