You will not be doing yourself any favors by training when you are too sick to be productive. Both resistance (weight training) and cardiovascular training obviously put a lot of physical stress on the human body. Continuing to work out when you’re seriously ill could make you sicker and/or prolong the length of your illness. Training when you are too sick can also have negative effects on your bodybuilding efforts—both physically and mentally. If you are significantly weaker due to your illness, you’ll have difficultly training with weight heavy enough to truly overload the muscles. You simply will not be able to train in the manner in which you have progressed. In fact, you’ll be taking steps backward. Mentally, training when you are too sick could cause your intensity and concentration levels to suffer greatly. You definitely wouldn’t want that less-than-all-out workout mentality that you were forced to use when you were sick to carry over into the future when you are well.
When you are taking time out of the gym to get better, you should still eat nutritiously and consistently. In fact, eating the way you should to augment your bodybuilding efforts may be even more important at this time. Eating well can help your body recuperate sooner. You should also be sure to drink plenty of water. Whatever you do, don’t use being sick or taking time out of the gym as a convenient excuse to go on a junk food eating binge.
That being said, I also suggest that you always push your body to its true physical limits. As you know, there are many different degrees of being sick. You never want to sell yourself short. I’m sure that you’ve had many instances when you thought you were too sick to work out—but went ahead and trained anyway. If you are like me, many of those workouts turned out to be very productive ones. Training even though you are sick—but not too sick—can have both physical and mental benefits as well. Physically, you can keep your training momentum going—instead of breaking it with time out of the gym. Mentally overcoming the challenge of being sick and continuing to train intensely and effectively can be a great psychological boost. That psychological boost can help propel your bodybuilding efforts to a higher level. You may feel the positive effects of overcoming the challenge of training when you were sick for many months in the future.
I suggest that you try to “tough it out” for at least a couple of training sessions. Your mind can accomplish some amazing things—but only if you ask it to do so. If, after a couple of workouts, you are not feeling any better, your strength in the gym drops significantly, or your intensity level and concentration level are not nearly where they should be, take a few days off. Without any guilt, second thoughts or reservations, just rest for a couple of days and reevaluate the situation later. Give your body the time it needs to get better.