First and foremost, if you go to a gym to work out, the answer is never. It is never ok to drag your sniffling ass in there and infect every machine, bar and bench in the place. If you’re going for a run, or you have a home gym, go for it. You aren’t showing how tough you are by going to the gym sick, you are being selfish, and putting all your fellow gym goers at risk.
As explained in this video, the general rule of thumb is the “above or below the neck check.” If your symptoms are all above the neck; couging, sneezing, sore throat, congestion, you can probably still work out. If you have symptoms below the neck; vomiting, aches, fever, chest congestion, you’re best off skipping the gym. The key is listening to your body, if you feel like you need rest, going to the gym probably won’t do you any good. It’s better to miss a workout or two than it is to prolong your illness or injure yourself.
If you decide to go for it, it’s best to reduce weights, reps and sets and the overall extent of your workout so your body can retain enough energy to continue fighting the illness. Avoid going to failure, testing one rep maxes, or anything else that is very strenuous. Stay well hydrated and don’t be afraid to go home early if you just aren’t feeling it.