Take Rest, If You're Sick
Some exercise enthusiasts find it difficult to suspend their fitness activities even when they're sick with a cold, flu, sore throat, or fever. Give your body a break. The risks of exercising at this time outweigh the benefits.
A fever is a stress to the body. Some studies have shown that adding to that stress by exercising may prolong the illness. So let your body recover and repair itself before resuming exercise.
If you have a cold but not a temperature, you should still avoid exercise. Exercise increases blood circulation and by doing so, may spread the virus or bacteria responsible for your misery to areas it wouldn't ordinarily reach. Also, your lungs may not be working efficiently when you have a cold, so your exercise capacity drops. A day or two of rest will do you more good than sticking to your workout schedule.