His timing is interesting because I, too, am struck with some form of sinusitis and am struggling with the question: am I well enough to go for a run?
The generally accepted rule of thumb is if you’re sick from the neck up (head cold, sore throat) then you can run. But if you’re sick from the neck down (flu, respiratory virus, other unmentionable nasties...) then you should not run.
All of that said, I'm now of the opinion if you have to ask yourself that question you should take the day off from working out.
After a weeklong domestic road trip followed by a weeklong stint in South America followed by a weeklong malaise, I thought myself well enough to get in a 15k. I had to cut the run short and have relapsed into my sniffling and hacking ailment.
If, like in The Ubster's case, you're scheduled for an important long run in a calculated training plan then rejigger the training plan by shifting or dropping a mid-week recovery or tempo run. Otherwise a couple of days off will not ruin your fitness and may actually do you some good (muscles get broken down during workouts and are only rebuilt on rest days).
So if you're asking yourself "Am I too sick to run?" then it may be time to trade in the Gu for some chicken soup!
"The part can never be well unless the whole is well."
-Plato