The disease usually strikes adolescent girls who have no reason to diet. They are not overweight. They are not preparing to take part in specialized sports activities requiring a slender figure. They have not been told to diet by their doctors. They stop eating because, in spite of all evidence to the contrary, they believe they are fat. Determined to lose the imaginary excess poundage, they refuse to eat more than a few morsels of food. Usually. the weight loss is rapid, sometimes more than fifty pounds in a few months.