This is a wonderful question. One that I can't answer, because I don't work out when I'm sick (Then again, I love having excuses for slacking
). I believe that if you take your body's reserves and energy away from healing, you won't get any better.
HOWEVER, if you're like my husband, you're only sick for two days because you're in such good shape.
He says: "It's not always easy. Just do it."
Wow, I know... that's great advice.
Perhaps the best advice is to read your body, and wait until you're in excellent shape to work out EVEN WHEN YOU'RE SICK