Anorexia is an undereating disorder that is caused by people viewing they are severely overweight despite being underweight/average.
A lot of overweight people claim to be anorexic, which is almost impossible.
It causes really low creatine (muscle mass) and often gives people a high metabolism, making it much, much harder for them to gain weight.
I got Anorexia when I was 7, and then I recovered a few months after being hospitalized at the age of 10. Sadly, it came back.