Is anorexia a mental illness?

Ok I relize that question may sound completely dumb but bear with me. When went through my health history for my daughters new pediatrician I was asked if there was any mental illness in the family, I said yes, I have anorexia. And the nurse laughed (a little too hard) and told me that wasn't considered a mental illness. So now I'm confused, because it was my understanding that it was. So perhaps an eating disorder is a whole other catagory in technicality? Does anybody know?