Easy like a fifty foot fall.
It may not leave, but you can fight it. Walk/exercise, be out in the sun.
Depression is a horrible thing. I myself had been fighting it a bit. It is difficult. What Ehkos Miku said is what i will say: go out for a walk, try to do something to take your mind off you know. Would you happen to know, or have an idea, as to what is causing this?