I'm fairly certain that at times I suffer from depression. I use the word "suffer" with a grain of salt, because I think that in most cases it's somewhat of a fake affliction. There are days in nights in which I feel crappy emotionally for no easily discernible reason. Sometimes through effort I'm able to pinpoint the cause, usually some insignificant event subconsciously blown out of proportion, and other times I am not.
The internet has a variety of numbers that estimate how many adults have a diagnosable mental disorder like depression. I've seen the figures range from 10% to 25%. It's not an uncommon occurrence. It's also not really something I would ever seek medication for. Like I said, I don't feel it's worthy of such attention. Even the times when I cannot pinpoint a cause don't alarm me. I'm smart enough to realize the irrationality in what I'm feeling, and am always resolute in the knowledge that it will pass in time. Usually in a day or less it does. (I think a lot of the time it may be brought on by a sleep cycle that is inconsistent during a good week.)