If you go to the Forum "Ask Andreas" I think that he is the one who has an answer for you.

Rule of thumb...if you have a chronic illness...go ahead and flush...if you have an acute illness...wait until you're well. When you have a bug your body is already working hard to heal itself...no need to add the extra burden of the flush. Others have successfully flushed when they had colds, flu, etc....but I'd wait til you feel better!