The illness is as old as humankind I suppose. Though what we often refer to by that term is not an illness but a temoprary state of our soul that doesn't stay longer than a day. I had deep sad times in my life that lasted much longer than that and I consider it still not an illness since I knew people who were seriously more ill than me suffering from a deep depression with their body also affected severely. One girl lost the ability to walk due to depression. I've been a nurse for some time. So I consider myself lucky that my sad times did leave some marks and scars but nothing permanent. The source of my sadness almost always is the fact that I feel so different from most people and need the company of my own kind so much but there is only now and then a person who understands. So basically my depressions are solitude sadnesses.