I have learned the hard way about riding when sick. I get to a certain point and I know that a cold will go into my chest. Sometimes riding outside will make me feel better, but then hours later I get worse. It's just hard for me to go slower than usual when I'm outside.
What I do is go for a brisk walk or go to yoga if I'm sick, but not too bad. That way, I feel like I've done something. Of course, I'm not training for anything. But, I've given myself pneumonia and bronchitis in the past, because I wasn't willing to rest when I should have.