I've had an okay experience working out sick. As long as it's "above the neck" meaning like headache and maybe nasal congestion, you could probably have an okay workout. Otherwise (nausea, bronchitis, whatever) probably wait.
Side note: I actually added like 12 pounds of muscle last semester while I unknowingly had mononucleosis (mono)! In retrospect it was obvious something was wrong, but I was totally oblivious at the time. I was surprised as all heck when I finally got my blood tested, but it goes to show you what's possible.
-casualbb