I am a masters runner and don't play with fate. Running with a flu could be fatal (heart damage) Cold could turn into something dangerous. The immune system gets distracted without rest. Its like overtraining or a strenous long run. Most colds covered by two weeks. Won't loose fitness in this period, could even come out fitter afterward (proper recovery). One or two cold/flu break a year is not something to panic about. Listen to your body.