I'm a novice weight lifter, currently going to the gym 3 times/week for an hour each time. I recently came down with a head cold. I know that if I go to the gym I won't be able to hit my usual max performance, but is it better to get in a few bad workouts or skip it entirely and just wait until I recover?
I can speak for myself from my own experience here and I would really recommend staying at home until you are completely recovered.
If you work out while ill, your body won't be able to give maximum performance, as it's fighting with the infection, and it won't be able to 'gain' new strength, for the same reason.
If you work out when your illness is almost gone, you are risking getting it back easily, as your body is still not completely recovered. You'll feel when your illness is completely gone and you are fully healthy. You'll know that you are ready to work out again.
Physically, your body is capable of performing to almost its maximum even when ill, of course. This comes from the ancient times when people were living more "in the nature, among the animals", where even when ill, you had to be able to fight or flight, which can still be applied in some life situations today (coming accross a bear, for example), but this is applicable only in life-or-death situations, which can be triggered by the survival instinct, especially by the adrenaline. So basically, your body is capable of great performances even when ill, but not for the reasons you want it to. There's no harm at stakes, but there's no improvement at stakes either. You will only make it harder for the body to fight the illness. It's not worth it.
So stay at home, rest, drink some good hot tea with lemon and wait until you are completely healthy, feeling strong again. And when you are, hit the gym and you will enjoy the fact that you are healthy again, working out happily as never before!
I think you should keep working out -- even increasing the intensity might help. I asked my doctor about this same question and was pointed to some research which says that there is no real performance loss and no negative impact on your recovery. Positive impacts may be increased emotional well-being from working out.
In the past, when I have taken off time I got into a downward spiral of losing strength and feeling worse -- adding to the overall misery of the cold. Lately I've changed my views and kept working out -- It felt so much better.
The rule of thumb I follow is if it is below the neck, rest, if it is above keep working out.
One article that was helpful to me is entitled Sports, Exercise, and the Common Cold:
David Nieman, Ph.D., who heads the Human Performance Laboratory at Appalachian State University, and has run 58 marathons and ultras, uses the "neck rule." Symptoms below the neck (chest cold, bronchial infection, body ache) require time off, while symptoms above the neck (runny nose, stuffiness, sneezing) don't pose a risk to runners continuing workouts.
This view is supported by research done at Ball State University by Tom Weidner, Ph.D., director of athletic training research. In one study, Weidner took two groups of 30 runners each and inoculated them with the common cold. One group ran 30 to 40 minutes every day for a week. The other group was sedentary. According to Weidner, "the two groups didn't differ in the length or severity of their colds." In another study, he found that running with a cold didn't compromise performance. He concluded that running with a head cold--as long as you don't push beyond accustomed workouts--is beneficial in maintaining fitness and psychological well-being.
Definitely wait until you're feeling better. It's better to give it your all when you're feeling better than to half-ass it with a cold just for the sake of keeping up the momentum. Occasional breaks during a training cycle can actually be very useful if you end up hitting a plateau.
Stay home and hit it with all you've got when you're better!
As the other answerer says, you should take rest if you're ill. But it wouldn't hurt if you do light exercise, such as 1/3 or half your usual intensity, so that it wouldn't hurt your body's effort to fight the disease.
Don't count on these exercises improving your strength or whatever your goal is, but when you're recovered, these help you adjust your exercise back to normal smoothly.