it really just depends. If its a little sniffle, I'd cut back on the intensity of your weight training, as its harder on the immune system than cardio recovery.
Of course, if you have a hacking cough, cardio isn't gonna be a good idea.
Basically, if feel pretty sick, take a few days off and keep eating good and get extra rest & fluids. If its just a little sniffle, lower intensity a bit, and you'll probably get over it before it has a real chance ot make you sick as a dog.