This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
"The Running Book" By the Editors of Consumer Guide
Recommended schedules should be followed as faithfully as possible, but not
blindly. There are certain times when you have no business running. If, for
example, you have the flu, a cold, or some other ailment, don't overexert
yourself and possibly harm your body by trying to run. If you feel a cold
coming on, however, running may help you get rid of it. But if you try this
cure, follow Dr. Kostrubala's recommendations. He suggests that you dress
warmly, take two aspirin in a glass of milk, and then go out for a run. Jog
slowly and see how you feel. Continue jogging until your body grows warm,
even hot, Then try to keep your temperature at that level.