The most important point to consider in the debate on whether depression is a disease is that it is indeed categorized as a disease by the DSM, the Diagnostic and Statistical Manual of Mental Disorders, which is the standard for naming and classifying mental diseases by the American Psychiatric Association (DSM-IV).
The DSM includes listings for all those elements of a disorder which qualify depression as a disease -- diagnostic features, specifiers, recording procedures and diagnostic codes, associated descriptive features, associated laboratory findings, prevalence, course, and familial pattern. Anyone who offers the view that depression is not a disease must confront the findings of the American Psychiatric Association and that organization's inclusion of various types of depression as diseases.
For example, Major Depressive Disorders include subcategories of mild, moderate, severe without psychotic features, severe with psychotic features, in partial remission, in full remission, chronic, with catatonic features, with melancholic features, with atypical features, and with postpartum onset.
The second most important point to recognize with respect to depression and its classification as a disease is that depression does not refer to simple sadness, which every human being experiences, perhaps every day, to varying degrees.
To compare clinical depression and mere sadness is to compare an ulcer with an upset stomach. An upset stomach will go away of its own accord, or can be alleviated with an antacid. An ulcer requires medical intervention. Similarly, mere sadness will go away of its own accord, or it can be alleviated with a talk with a sympathetic friend or a bit of good news. On the other hand, clinical depression requires therapeutic and/or medical intervention. Depression is a disease which does not simply go away of its own accord, but requires substantial and specific treatment. Depression will not go away after ...