So, when should you run when you're sick? The general rule of thumb is to go by your symptoms. If you have symptoms just from the neck up (sneezing, nasal congestion, but no fever or chest infection symptoms), chances are you're ok for some easy running. Notice I said easy. Don't expect to run at your normal pace, intensity or even mileage, but you can run without really making things worse. If your symptoms include a fever, back away from the running shoes. This is a sure sign your body is working very hard to get rid of the infection and running will only make it worse. If your symptoms include chest congestion, trouble breathing, deep cough or more, you should take some time off and rest. And running is not rest.
Have the flu? Forget about it. Really, if your symptoms are making you feel badly, running is only going to prolong your sickness. But if you're well enough to go about your day, not take time off work, then a short, easy run is probably ok. Use how you feel as a guide. You know how running through an injury usually makes the injury worse? Same goes with illness. Your body needs time to get through the illness, and recover from it.
As the doctors always say, make sure you drink plenty of fluids while you're sick. This is especially important if you take any decongestant because, by nature, that type of medication will dry you out. Drinking plenty of fluids will help keep you hydrated and your circulation moving to get rid of whatever bug bit you. Same goes for your nutrition: aim for eating as cleanly as possible. Research suggests that refined sugars can suppress your immune system, so try to avoid processed foods and sugary drinks while you're under the weather.
Once your symptoms are subsiding and you've gone through a 24-hour period without feeling like complete crap, it may be time to don the running shoes and hit the road. Again, don't expect to run like you normally do. Take a day or two to run easy and then start getting back into your normal groove. Trying to get back into things too quickly can lead you right back to your sickbed, especially if you had the flu or other multi-system infection.
No runner really likes to take a break from running, even when sick. But taking a few days off to rest and get well may be the difference between a short-lived sickness to a full-blown, down-for-the-count sickfest.