Google depression linked and these are the results you get. Depression seems to be linked to everything:

• Depression is linked to cancer, no?
  
• Depression is linked to concussions, ouch, really?
  
• Depression is linked to early menopause, Parkinson's, facial pain....
  
• The list is endless... 

Depression has become a very dangerous term.  It's a catchall phrase like cancer.

Cancer is a word that is used to describe about 110 different diseases caused by the uncontrollable multiplication of cells.  And just like cancer, where multiple genes and pathways are involved in even the most basic forms, a wide range of factors cause depression. 

Don't get me wrong, depression is a very serious mental health issue.  But the lack of further differentiation creates significant misdiagnoses and the prescribing of drugs that don't work.  Anti-depressant effectiveness is 50%, at best.

Accelerating our understanding of mental illnesses requires further developments in neurotechnology. In time, neurotechnology will help define mental illnesses from the bottom up as brain imaging technologies illuminate the neural substrate of disorders.