If you have a cold or fever, you will certainly not be fit to run in a marathon. First you will not even leave your bed as you will be feeling miserable. Fever lowers your body's immune system and your body will feel tired. Your body requires rest so your immune system can fight the fever causing bacteria. if you take to running a marathon, you will end up either fainting midway or just collapse the moment you take a couple of steps. Having a cold is even worse as running will induce you to sweat more and aggravate your cold. Colds are also irritating as running noses have to be kept under check all the time.