Well, I should say disease resistant. Nothing is disease free.
I few years ago, I bought a thin emaciated female mandarin. The gill bone was bare with algae growing on it. I got her for 5 dollars. I don't rescue fish from LFS or Petco. If there is a sick animal that I think I can keep alive and do well, I will buy it at a discount. I will not pay full price for it. Rescue animals from LFS is the most misguided action a reefer can do to help the fish. If the LFS cannot take care of an animal, they should not get it. If they get it, they must loose money, or else they just keep on order animals they cannot take care of if they keep on profit from it.
Anyway, after several months this is the fish.
The gill plate on the L side still have hair algae growing on it. This did not heal for a year. Now you never know she was almost dead. I did not take picture of her when I first got her, but I make a point to take these type of pictures from now on.
How is that for disease resistant. Is there any animal with plant growing on their bare bone and live?