sick as a dog?

Does anyone know where the phrase 'sick as a dog' comes from?
I ask because I've been sick for a week now, and while I have felt lots of things (exhaustion, inability to breathe properly, soreness, etc.), I do not believe any of them are dog-like.  I mean, I'm not a pet person (or a dog), so I can't be sure, but the dogs I've seen never seem this miserable.  Maybe I've just never seen a sick dog?
Anyway, day 4? of lying in bed and being pathetic is starting to get a little old.  I'm hoping to be back among the living in another day or so.