Thanks for the response Neo.
You mention the mind/body connection and this no doubt is a huge contributor. You could have a perfectly healthy person and if they go through a period of strong "shock" trauma for example the death of a loved one, and their health can rapidly deteriorate. Low spirits over an extended period of time.. same thing. Depression can be caused by chemical imbalances in the body as well as outside influences. Interestingly, I've recovered significantly from candidiasis symptoms (and the sub-par digestion that goes with it) in recent weeks, and very noticeable improvements are a more mellow attitude and a major easement of depressive emotions! I'm generally in better and brighter spirits and this has happened synchronously with the physical improvement. I think it's 'circular'- the body's physical and emotional/psychological sides each influence the other.