Kind of depends on the sickness/symptoms. If I've got a fever, aches, and chills, it's best just to totally rest. A faster recovery is better than a bad workout with a long recovery. If I'm just congested, coughing, etc., then i can probably still get in a great gym workout, but pushing cardio could lead me to develop pneumonia or damage my lungs. A stomache issue is also probably best to rest, because intense activity will probably worsen your symptoms. The only sickness that you should definitely workout while having is a mental illnesss.