Most experts agree that, if your illness is 'contained' above your neck and doesn't include a fever (think common cold or allergies) then chances are doing a little moderate exercise will do you no real harm. Although working out won't make your cold or allergies go away any quicker, it won't make them worse or prolong your sickness either. The key thing to remember here though is to keep the word 'moderate' very much in mind. If you want to do cardio then go for a walk rather than a run, and reduce your weight-training load by around 25 percent if you can't tear yourself away from the iron. Be sure to stay away from the gym if you have an illness that can be passed onto others, though.
Retreat And Regroup
If you're suffering with flu-like symptoms or are experiencing discomfort below the neck, then err on the side of caution and take a break from working out until you feel much better. Illnesses of this kind, especially those which proliferate aching muscles, fever, fatigue, swollen glands and nausea, tend to batter your immune system - so your body needs as much energy as it can muster to defend itself and eventually promote recovery. Working out will use up a great deal of that much-needed energy and slow the recovery process down considerably, thus prolonging your time stuck out on the side lines. Time to rest up.