I haven't worked out all week long. On Monday my allergies grabbed me by the face and have since refused to let me go. Today is the very first day I've felt even remotely human again... and that's still quite a stretch.
Technically, I could've worked out this week. My rule of thumb is that as long as it's not out of my head, it's safe to work out. Head colds, sinus infections, even maybe a tickle in the throat. Those are safe. You might want to dial the intensity down a few notches, but you should be alright.
If it's anything lower, though, I think I would stay away from working out completely. If you can't breath properly because you have bronchitis..... just take it easy.
Me, personally? If I'm feeling too cruddy to go to work, or just entirely too foggy-headed to think straight, working out is completely off the table. And the main reason for this is that I want to give my body the chance to rest. I want it to heal as quickly as possible so that I can get back to working out at my full intensity.
What can you do to keep yourself healthy while sick? Nutrition! Seriously, I know how I am when sick. Food is the last thing that is on my mind. But you have to keep yourself hydrated (I tend to stock up on G2 - Sugar Free) and feed yourself healthy snacks. Fruits, veggies and healthy proteins. I know that when I finally started snacking with watermelon, carrots and boiled eggs, I started perking up almost immediately.
Truth is, the lack of food in your system could be lending itself to the "fog" that a lot of people feel while sick and congested. The smaller the portions and more frequently I ate the clearer I felt. Give it a try next time you're feeling under the weather. See if it works for you too!