I have asked my T about this and he does not want me to cancel as long as I am physically able to make it to his office. I have to pay for all cancelled sessions that can't be rescheduled in the same week (regardless of how far in advance I cancel, and regardless of the reason) but I don't pay for the sessions I go to, so I have absolutely no incentive not to go. And since T has explicitly said that he is not bothered by the risk of contagion, I trust that he means it. I don't mind it when he works despite having a cold - on the contrary, I very much appreciate the fact that he comes in anyway. We don't shake hands when either one of us has a cold, though.