After reading the Science and life cycle I understand . I also think the immune system of the fish has improved since its been in my tank.
it's not that the immune systems 'improve', exactly-it's that the fish's health starts rising from sub par after it's gills start functioning so it can breathe easier so the immune system CAN simply function at norm, and that the infection drops to a level the immune system can fight off. the healthiest fish in aquaria can (and often do) get ick. it's a simple numbers game. w/ enough soldiers and a relentless assualt, the tiniest creature will always overwhelm a much larger sized 'foe'.
(like fleas on a bear, or russia in afghanistan,heh)
the next time conditions are right for ich reproduction, your fish will likely get it again, and the 2nd time it's more likely to kill, not less likely. (in most cases-and a boosted immune system should mean for the most part a highly reduced chance of a 2nd infection to begin with, heh).
one certainly can't make a fish's immune system 'ich', or anything proof, especially when there will come a time when the best immune system stops functioning at 100% temporarily, from some type of shock to the fish's body, (which is THE opportune moment for most baddies) nor boost it beyond it's genetically pre-determined capabilities
ich's 'programming' is very much 'in tune' w/ fish life on the reef in that it's fairly 'proximity dependent' for its infection spreading mechanism between individuals-if it wasn't distance dependent, there wouldn't BE any reef fish, heh.
yet it manages to 'catch' fish from time to time on a reef, where both distances and interfering currents, volumes of water are orders of magnitude greater than in our impossibly teeny glass boxes. how difficult does anyone think it could be for ich to easily get a hold in and wipeout even a 4k gallon tank ?
hth