One day in March 2010, Isak McCune started clearing his throat with a forceful, violent sound. The New Hampshire toddler was 3, with a Beatles mop of blonde hair and a cuddly, loving personality. His parents had no idea where the guttural tic came from. They figured it was springtime allergies. Soon after, Isak began to scream as if in pain and grunt at his parents and peers. When he wasn’t throwing hours-long tantrums, he stared vacantly into space. By the time he was 5, he was plagued by insistent, terrifying thoughts of death. “He would smash his head into...