The general rule is if the cold is above the neck then run, once it hits your chest, don't.
It takes at least 10 days to lose any significant fitness.
Running increases your body temperature, and can compromise your immune system (especially the first day after strenuous exerciser) which can aggravate your cold or flu symptoms , delay full recovery or even open you up to other viruses and infections.
Besides which, its not fun to run when you aren't feeling well, and remember running
is supposed to be FUN!!!