The golden rule is don't train when you are not well. Your training will not be effective and will most times just hinder your recovery. We suggest taking it a stage further. If you are unable to train because you are unwell for more than a couple of days then don't run on the first day you feel ok. Have another day of rest. Runners inevitably think they will lose fitness every day that they don't train but recovering properly will see you running better a week later.