Mild, or self-limiting or otherwise transitory. I know (closely) three people in two different countries who have a 15/100,000 disease that is self-limiting.
It wouldn't be very surprising to learn that every single person has some kind of genetic defect -- perhaps dozens -- but usually the compensatory mechanisms are strong enough to mask them, at least while you're young and strong overall. It makes a lot of sense that health should be a spectrum or a kind of dynamic equilibrium, rather than a binary on-off condition.