If you have fever or aches and pains DO NOT RUN! Lost count how many times I've posted this over the years, but if you have a head cold you are ok to run. If you have flu or other illnesses, a definite no-no, you run the risk of an inflammation of the heart muscles which can lead to you never being able to exercise again or at worst, even death. I knew a very fit 25-year old in the Marines who ran 10km when he had not recovered properly from flu, and he dropped dead 30 mins after getting home from his run. I also know of 5 others who are unable to run at all any more after running when unfit. PLEASE don't do it! Rememebr the rule - symptoms above the neck, you can run, symptoms below, don't.