I've never canceled because of being sick, but only because by the time my session rolled around, I felt well enough to go. There have been a few times where my voice was nearly gone, but with my allergies, that happens, and I just deal with it. T has canceled my in person appointment and we did a phone session once because she was sick.

I think, if either you or the T is contagious or simply feeling so unwell that you can't concentrate on the session, the most respectful thing to do is cancel, or consider a phone session if you feel well enough to talk on the phone.