Looking back, Denise Watkins is convinced a sore throat disabled her son one year ago. His bizarre obsessions emerged not long after, first the constant nightmares about snakes and alligators, then the relentless hand washing that left his skin raw and chapped. Mrs. Watkins, who lives in Lakeland, Fla., was told that her son, Will, had obsessive-compulsive disorder. But it seemed odd, Ms. Watkins thought. Will was only 5 years old, and his illness seemed to burst out of nowhere. "In a matter of weeks," she said, "this was a totally different child." Then one day, buried on a Web...