Take Alzheimer's disease. It’s true that it is diagnosed mostly by analyzing human behaviour, since the causes for Alzheimer's disease are not properly understood.
Although it looks like a “mental disease” because of the methods of identifying it, the difference between Alzheimer's and, let´s say, ADHD is huge. ADHD is a pattern of behaviour with no exact bio-chemical correlation, while Alzheimer's: “Although the causes of Alzheimer's are not yet fully understood, its effect on the brain is clear. Alzheimer's disease damages and kills brain cells. A brain affected by Alzheimer's disease has many fewer cells and many fewer connections among surviving cells than does a healthy brain.“
We don´t say that people with Alzheimer's are absent-minded and try to change their behaviour, but we instead look for a bio-chemical cause of their memory loss.
Alzheimer's:
normal