[OMFG!You'reActuallySmart!]

It can be a disability depending on how adversely affected you are. However, depression is the most common mental illness, so I'm unsure if it would be considered a disability. Anxiety disorders are being considered as a disability more at least in Canada when PTSD when finally considered one in the year 2000, partly from Retired Lt. Gen. Romeo Delaire (I think that's spelled right). Of course, this doesn't mean everyone with PTSD suffers from a disability, it means they are eligible for consideration.

So basically everything (or nearly everything) counts on a case-by-case basis.