Anorexia Nervosa (AN) is an eating disorder which currently affects around 3 million people per year.
Anorexia nervosa (AN) is a serious mental illness categorized by a failure to maintain a minimally normal weight, a fear of gaining weight or becoming fat, and preoccupations about body shape or weight. AN is associated with significant morbidity and a mortality rate as high as that seen in any psychiatric illness. Biological factors, including genetic predisposition, appear to play a role in the development of AN.
Source: Anorexia Nervosa: Current Status and Future Directions Annual Review of Medicine, Vol. 61:425-435
The causes of AN are currently unclear - but genetic, occupational and cultural factors are known to play a part. There are treatments available (e.g. CBT), but there is no conclusive evidence that any particular treatment works better than others.