doctors agree that we should use the ‘neck rule’. If your symptoms are below the neck (chest cold, bronchial infection, body ache, intense coughing, vomiting, diarrhea, etc.) you should take time off. If, however, your symptoms are above the neck (runny nose, stuffiness, sneezing, etc.) you can carry on running, but watch out for dizziness, nausea or profuse sweating. If you feel any of these symptoms, stop!
Take extra care when training with anything worse than a minor cold because it could escalate into more serious conditions, such as sinusitis, and never run if you have a fever or flu. Wait until the day after your symptoms disappear and then go for a short, easy run. Wait one to two weeks before resuming pre-illness intensity and mileage, otherwise you risk a relapse.