If you hit the weights hard when you know you're already sick you'll probably end up much worse off. I'm even considering the policy of not working out when someone else in the house is sick (recently got a stomach flu, it was probably unavoidable, but it hit me the same day of a workout and I suspect if I hadn't worked out that day I either wouldn't have gotten it, or it wouldn't have been nearly as bad... alternately if I hadn't eaten anything for about 24 hours before that day, it also wouldn't have been as bad... grrrr.... stupid stomach flu).