Since cold and flu season started a lot of folks have asked me if they should work out if they're sick. Well, a good rule of thumb when figuring out the answer is the neck check. Exercise is generally considered okay if your symptoms are all above the neck - sore throat, runny nose, sneezing, congestion. Light cardio might actually help reduce these symptoms and give your immune system a little boost.
But if your symptoms are below the neck like fever, chest cough (with lots of phlegm) or nausea you should probably choose your couch over kick boxing. Exercising can make those symptoms worse.
So check in with your self and listen to what your body says. If you are just tired or in the throes of PMS a work out could really do you some good.