CHICAGO - They call her "Ana." She is a role model to some, a goddess to others — the subject of drawings, prayers and even a creed. She tells them what to eat and mocks them when they don't lose weight. And yet, while she is a very real presence in the lives of many of her followers, she exists only in their minds. Ana is short for anorexia, and — to the alarm of experts — many who suffer from the potentially fatal eating disorder are part of an underground movement that promotes self-starvation and, in some cases, has...