This reminder sounds a bit strange but it is true. When you feel like you are getting sick, it might still be OK for you to run. I refer to Runners World for most of this insane argument, but I am happy to say that you can still get some good miles in even if you are coming down with a cold. If you are bed ridden, then running is probably out of the question.
Take a read of the article and make up your own mind. I'm running.