This is a good question, I have wondered before myself. Here is my opinion. While you can count it as a backoff week, I don't think you would get the same benefits as a regular back off week. What I mean is for a regular back off week, you normally hit the end of a heavy cycle, do some light/moderate work, to let your joints, tendons, muscles, etc. recover and then start your next cycle fresh.
On the other hand, if you take a week off, from say flu or a bad cold, chances are you aren't starting your next week off "fresh". More likely than not you may have even backslid a little.