*Wills the internet connection to not switch off again this time so she can actually post*
I think it depends on the kind of reading you're doing. I would not, for instance, do a 2 hour Tarot session for a paying client if I was ill in the slightest. Anything over and hour of reading, and I begin to get fatigued and sluggish. I would probably also refuse short readings for friends when ill - though that's usually just an excuse for when I just can't be bothered to read that evening! (Yes, it's true - sometimes I can't be bothered to read Tarot!)
However, I don't mind reading for myself when I'm ill. It takes less concentration and effort for me, since I'm not counselling another person at the same time (which seems to be what fatigues me most in a reading). I find it also provides a fun distraction from the illness - I detest being bored when ill, so need to find things to do!
As for illness affecting the cards... I am of the school of thought that doesn't believe in energies affecting the cards and therefore their usability at a later stage. Just as using a pen when ill won't make that pen write any worse, I don't think using a Tarot deck when ill will make it more difficult to use later. My experience has seemed to back this up so far.
Blessings,
Kiama