When are you too sick to work out or play? Any time you have a fever, are vomiting, or are dizzy. If you feel like you want to get some activity then you probably can. Make sure to drink plenty of water and drop the intensity down several notches. If your body is fighting off an infection, it needs all its energy to devote to that. Any activity should not be too taxing to your system.
A short walk to get some fresh air and sunshine usually does wonders!