The general rule of thumb is if your symptoms are from the neck up you can workout.  If your symptoms are fever, chest congestion, flu like then probably not. 

 

Keep in mind that even if your symptoms are from the neck up and you choose to exercise go with how you feel.  You may not be able to exercise at 100%, but studies show that a light workout, maybe 50% of normal intensity can help boost your immune system and may help you feel better.

 

Attached is a more in-depth article on the subject.