It depends. If I have a sniffle, I find that a workout helps to open my nasal passages. If I'm hacking up a lung, I take it easy for a few days. It's kinda hard to exercise when you can't breather properly.
How long did the doctor tell you you'd be out of commission because of the shingles ? I know you want to get back into a groove, but you don't want to start working out while your immune system is still weak. Trying to exercise might just make you feel much worse and could cause you to be sick for longer. The right amount of exercise will keep us fit, too much can break down our immune system.
Try not to worry. When the weather as well as your health improves, walking would be a good way to slow ease back into a routine.
Mostly, don't beat yourself up because you can't exercise. you do have a legitimate reason. You're not a slacker because you need to take some time off.