She was admitted to the hospital and during that first day became confused and began to speak in baby talk — something she hadn’t done for years. The M.R.I.s of her brain and spine were dotted with cloudy blotches that indicated swelling and inflammation. She was diagnosed with a rare autoimmune disorder known as acute disseminated encephalomyelitis (ADEM). In ADEM, antibodies mistakenly attack the myelin sheath that protects the nerves in the brain and spinal cord, and in this case it was affecting the girl’s legs. ADEM is seen mostly in young children who have recently had an infection, and the girl was recovering from a bad cold when this happened. Once her doctors felt certain that she didn’t have an infection in her brain, she was given steroids. Within 24 hours she was no longer confused and was able to walk. She was discharged on a two-week course of prednisone.