Avion was a healthy and athletic 15-year-old who became critically ill when he was admitted into a pediatric intensive care unit. For Robin Williams, assistant professor of pediatric hematology/oncology at the University of Minnesota Masonic Children's Hospital, Avion offered a medical puzzle she couldn’t crack on her own. His immune system was on overdrive and it was attacking healthy cells and organs within his body. Though testing ruled out blood cancers, it was only when Williams consulted a friend outside the hospital that she realized Avion was suffering from TAFRO, a subtype of the ultra-rare disorder idiopathic multicentric Castleman’s disease, a condition that has characteristics of both blood cancers and autoimmune disease. We spoke to Williams about the challenges physicians face in diagnosing patients with rare diseases, the thought process she went through in Avion’s case, and why she’s working to educate other physicians about the ultra-rare condition.