Depression has a socio-economic predeliction to it.  It is genre specific.  Sadly it is also very aggressive.  In so many ways it targets groups of people and creates symptoms and conditions that are remarkably similar, almost to the point where you think it is tactical in it's approach.  It also occurs in waves and varying degrees and whilst not all forms of depression are clinically determined, sometimes just feeling a bit low or down can be a precursor to a more devious beast that lies within.