If we look at physical illnesses we could put them on a spectrum of 'Happened spontaneously/no choice in the matter' to 'Was clearly created by something that happened in their life'.
From less causal, to more causal I'd suggest a list like this.
Genetic disorders. (Entirely genetic, from birth)
Down syndrome. (Genetic, but age of parents plays a part)
Cancers that happen 'randomly' (eg. breast cancer). (Random, but some people have genetic disposition)
Common cold. (Not sure where to put this on the spectrum, but caused by a cold, but some people are more susceptible than others).
Obesity. (Some people have a higher disposition to obesity)
Smoking related illnesses. (Caused by smoking, but some people more susceptible than others).
HIV. (Caused by a virus transferred by contaminated blood/unprotected sex)
A broken leg. (Caused by physical trauma)
That is - the illnesses at the top of the spectrum have some kind of genetic cause, or we don't know enough about them to say how/why they occur, whereas the illnesses at the bottom of the spectrum are clearly caused by some life event.
With a mental illness like schizophrenia - it seems clear that this would fit at the top end of the spectrum - the schizophrenic was schizophrenic from birth, and there was nothing that 'caused it'.
However, it seems that depression is often the result of life events - stress, addiction, pregnancy, grief etc.
Is this a valid way of looking at depression? What terminology and research is there about it?