I just read this article on Runner's World Web and thought it was great.
SHOULD YOU RUN WHEN YOU'RE SICK?
It's a long article so I'll sum it up for you...
One researcher/runner uses the "neck rule" - symptoms below the neck (chest cold, bronchial infection, body ache) require time off, while symptoms above the neck (runny nose, stuffiness, sneezing) don't pose a risk to runners continuing workouts.
But doctors say you should still use extra caution when training with anything worse than a minor cold because it can escalate into more serious conditions affecting the lower respiratory tract and lungs.
Also, if your temperature is above 99 degrees, skip your run.
Hold off on running until the day after symptoms of a flu or cold disappear, then go for a short, easy run. Runners should wait one to two weeks before resuming their pre-illness intensity and mileage. Otherwise, you risk a relapse.
BOTTOM LINE - obey your body and the thermometer.