Recent research suggests people with anorexia nervosa may lack a type of overconfidence bias.  The majority of people feel themselves more attractive than others would rate them. In contrast, people with anorexia nervosa seem to more accurately judge their own attractiveness compared to unaffected people, meaning that they potentially lack this self-esteem boosting bias.

There is no single cause for anorexia. It comes from a mixture of social, psychological and biological factors.

Psychological
Distorted body image
Poor insight
Self-evaluation largely, or even exclusively, in terms of their shape and weight
Pre-occupation or obsessive thoughts about food and weight
Perfectionism
OCD (Obsessive Compulsive Disorder)

Emotional
Low self-esteem and self-efficacy
Clinical depression or chronically low mood
Intense fear about becoming overweight
Moodiness or 'mood swings'

Interpersonal and social
Poor or deteriorating school performance - not present in some due to their perfectionistic tendencies
Withdrawal from previous friendships and other peer-relationships
Deterioration in family relationships

Physical
Extreme weight loss
Endocrine disorder - leading to amenorrhea (cessation of periods)
Starvation symptoms - reduced metabolism, slow heart rate (bradycardia), hypotension, hypothermia or anemia
Growth of hair "lanugo" over the body
Abnormalities of mineral and electrolyte levels in the body
Zinc deficiency
Reduction in white blood cell count
Reduced immune system function
Body mass index (BMI) less than 17.5 in adults, or 85% of expected weight in children
Pallid complexion
Sunken eyes
Creaking joints and bones
Collection of fluid in ankles during the day and around eyes during the night
Constipation
Very dry/chapped lips due to malnutrition
Poor circulation, resulting in common attacks of 'pins and needles' and purple extremities
Nerve deterioration - difficulty in moving the feet
Headaches - caused to malnutrition
Thinning hair
Brittle nails
Constantly feeling "cold"
Easily bruised
Dry skin

Behavioural
Excessive exercise
Food restriction
Fainting
Secretive about eating or exercise behavior
Possible self-harm, substance abuse or suicide attempts
Extremely sensitive to references about body weight
Become very angry when forced to eat "forbidden" foods

Psychotherapy and famly therapy are common courses of treatment. Forced weight gain is used in life-threatening situations.

Recovery is a touchy subject. Compliments from well meaning friends or relatives are often misinterpreted. When told how much healthier she looks, her mind replaces "healthy" with "fat."

This disease struck me when I was 13.

My parents first took me to a counselor. After one session, I decided that was not the route for me. My Mom then bought me a book that was instrumental in my recovery. It detailed the life of a young girl that was also struggling with anorexia.

Finally! Someone that I could relate to.  

The book is called The Best Little Girl in the World. The sequel, Kessa, includes therapeutic discussions with her psychiatrist. In the second book, Kessa's friend suffers from Bulimia Nervosa.