The best advice I have ever had when exercising when feeling ill is to follow the above/below the neck rule! If your symptoms are above your neck i.e. runny nose or sore throat then you can continue with your exercise although it is best to go at a lower intensity then usual. If your symptoms are below your neck then it may be best to skip the workout! Apparently studies have shown that exercising at a moderate intensity will not make your cold worse or compromise your immune system.
For more information on exercise and working out visit Jillian Michaels' website here.