When you are sick, take a break from exercising so your body can heal, and you can get better. When you are sick your body will work hard to heal itself. Even if you do workout, your body is focusing more on the healing process than on building muscle and endurance.
If you are having a hard time fitting in a workout, set up a time to get in some exercise and stick with it. Even if you only have a few minutes to commit to it, you will feel better knowing that you stuck to your guns and got some exercise.