A 5-year-old previously healthy child is taken from his day care to the pediatrician because his mother is concerned that the child has been fussy for the past few days and now has a rash on his face and torso . The mother also says the boy told her his head hu, and she thinks he may have a low-grade fever. On examination the child is afebrile and not ill-appearing. He has a rash extending over his entire body except for the palms and soles. The infectious agent causing this disease can lead to what complication in immunocompromised hosts?