I assuming you mean a typical cold or the like. The smartest thing to do is give yourself time to heal before attempting to attack the weights again. You will not be doing your training, your health or your training partners any favors by trying to rush back into the gym while you are sick.
Now the real question is, how do you determine when you are too sick to train? This is where things are not so black and white and the answer is very much relative to each individual and the situation.
Ask yourself how much will you really be able to accomplish in the gym that day. If you are at a point where you feel weak, tired, or have not been able to eat anything due to the illness, I doubt you will be able to achieve maximum overload and intensity during that training session. If that is the case, an extra day off to get back on your feet may be just what the Dr. ordered. In fact, training when you are sick can prolong the time it takes to get over your illness. Don't let your enthusiasm to workout cloud your better judgement.
You always need to look at the bigger picture and past just one workout. It is better to take it easy for a day or two and be back to full strength rather than rush back too soon having marginal workouts and potentially prolonging the illness or increasing its severity.
Another thing to realize, gyms and fitness centers aren't exactly germ-free environments. Chance are you probably caught your bug at the gym. You wouldn't want to train around someone when they are sick so don't expose anyone else while you are sick.
