Zencat

IMO depression is a disorder. Unlike a disease where a pathogen, be it a genetic or viral/bacterial can be, in a great many cases of diseases, traced to a most always organic cause.

Brain disorders often times when manifested in emotional or a cognitive causation can be a mix of organic
(brain structural/chemical rearrangement) through genetics and/or development maladjustment...trauma, maybe physical damage or emotional/ cognitive shock/abuse.

I think of a disease when there are specific symptom that follow in a course tissue degeneration that is generally much the same for all that are affected.

Alcoholism onset may be rapid or very lengthy in development. The symptoms may be masked by the individual making detection difficult. Try managing skin cancer in its latter stages...not likely, yet the alcoholic may be able to pull off work, be greatly mobile and higher functioning than a late stage skin cancer patient.

Diseases from what I know can be traced back to a very likely single cause. Alcoholism causation is many times manifest because of a great variety of experiences, personal history, distorted perceptions, family history and there is more causes that one can directly know.