Murphy's Law always seems to kick in when you get a cold. You're healthy all year long and then four days before that big 10K, you come down with a cold. In my case, I can almost predict it. I've often wondered if it might be related to my training. It's odd, because moderate amounts of running actually boosts your immune system helping your body fight off colds. But maybe I push myself too hard at the end of my training and end up lowering my body's defenses due to the extra stress. Seems to be some research supporting my theory. One study shows that after a hard workout session, you can lower your body's immune system for up to 9 hours following the workout. So, if you're exposed to some of the nasty colds going around shortly after having had a really hard workout, you might be more susceptible to catching one. Might be worth wearing one of those masks and slathering on the hand sanitizer after such a workout!
So, what do you do if you aren't feeling well. Should you run? I asked my doc, and he said checking your resting heart rate is a good method to help determine whether or not you should run. Check your pulse for a full minute. Be sure to do this while at rest (not after climbing a flight of stairs). Also be sure you haven't just downed a Starbucks venti espresso. If your heart-rate is a few beats faster than normal, this could be a sign that your body's fighting off and infection and you may want to hold off on that run. Taking your resting pulse on a regular basis (just after waking up) is a good habit to make. A normal resting heart rate can be anywhere between 60-100 beats per minute. You you need to establish what's normal for you. If you're a runner, don't be shocked if your regular resting heart rate is lower than 60. That's pretty typical.
My doc also said that the "head/chest test" is another good measure to use in determining whether or not to run. If your symptoms are limited to the neck up (runny nose, mild sore throat, and itchy, watery eyes), then it's probably okay to run, but at a lower intensity than normal. If your symptoms are presenting themselves in your chest or lower (heavy, productive chest cough, aches and pains, fever), then ix-nay on the unning-ray. You should be fever-free before running.