The truth is that you can't win in a situation like this, and you'll drive yourself bats trying to.

I say "Good days and bad days, like everybody" or "So far, so good", but usually I just say "Fine". If somebody REALLY wants to know, he or she will say, "No, how are you really?" And I tell them. But first I give fair warning and offer the opportunity to opt out.

My brother is rather of the persuasion that you just can't let yourself start THINKING you're sick, blah blah blah. So if he asks how I'm doing, I just say "Peachy keen, how are you?"