Im curious (not in a mean way - but genuine curiosity) - do you have an agenda? I don't get it - Its easy - just google - there are numerous articles discussing cryptocaryon immunity in fish - both specific - and generalized. Its not a question or a myth or anything thats debated. Fish have specific immunity to pathogens - including CI and velvet. It may be partial - it may be full - it may not last for ever - but it most certainly exists.I wouldn't call that an immunity. It's a resistance at best.
http://www.jimmunol.org/content/200/1_Supplement/53.19