A lot of runners continue running through colds and the flu, while others reduce their running. In my case, I go by my energy level. If I have high body-energy, I continue running. Otherwise, I reduce my running by taking more rest days. In fact, I modify my running by my body-energy all the time, colds/flue or good health. High body-energy means I feel good, energetic and am looking forward to my next run. Low body-energy means I'm tired, sluggish, and don't want to go out.

Hmm.  I answered this yesterday but I don't see it, so excuse me if this is a repeat.  I've been told that if your symptoms are above the neck you can run, if they're below then you shouldn't.  So if you have a runny/stuffy nose there's no problem but if you're coughing or your lungs are congested then don't run.  If you have a fever then running isn't a good idea because you'll be raising your core temp more just with the activity, and aches and pains will just turn a marathon into an awful experience.  If you decide to run don't expect to run your fastest.