Well, let's look at this a bit more objectively. The life cycle of this parasite is interesting and is important to understand when evaluating a treatment. The stage where the parasite is attached to a fish is called a trophont. The trophont will spend three to seven days (depending on temperature) feeding on the fish and that is what you see symptomatically when you see "salt sprinkled on the fish". After that, the trophont leaves the fish and becomes what is called a protomont. This protomont travels to the substrate and begins to crawl around for usually two to eight hours, but it could go for as long as eighteen hours after it leaves it's fish host. Once the protomont attaches to a surface, it begins to encyst and is now called a tomont. Division inside the cyst into hundreds of daughter parasites, called tomites, begins shortly thereafter. This noninfectious stage can last anywhere from three to twenty-eight days. During this extended period, the parasite cyst is lying in wait for a host. After this period, the tomites hatch and begin swimming around, looking for a fish host. At this point, they are called theronts, and they must find a host within twenty-four hours or die. They prefer to seek out the skin and gill tissue, then transform into trophonts, and begin the process all over again. What this means is that when your tank is infected, you can actually see symptoms during a very small part of the life cycle, and it why your tank is infected even though your fish are resistant. It will also explain why symptoms come and go.
Many hobbyists are fooled into believing they have cured their fish of the parasites, only to find Ich present again on fish a few weeks later; a reason why following through with a full treatment protocol is so important. Don't make this mistake and be lulled into a false sense of security. The parasites may be in a stage where they are merely regrouping and multiplying for their "next offensive." In the wild, this sort of massive reproductive phase ensures that a few will find a suitable host to continue on the cycle. In the close confines of our aquariums, though, it means comparatively massive infection rates.
This disease is usually associated with several environmental triggers. Changes in water temperature, exposure to high levels of ammonia, nitrite, or nitrate, low pH levels, low dissolved oxygen often associated with overcrowding, are all factors contributing to the onset of the disease. You could lump all of these in a general category of "stress", but it is more appropriate to think of all of these as "unnatural conditions". In fact, Cryptocaryon irritans is rare in the wild even more unlikely to be lethal. Ich is truly a disease that exploits the conditions of captivity to reproduce and easily find suitable hosts.
By the way, trophonts are under the skin so cleaner wrasses and cleaner shrimp have no real effect on reducing this parasite.
Now, there are three effective treatments: hyposalinity at 1.009, copper, and tank switching (which is infeasible for the vast majority of folks on this board). While it is true that some fish will eventually develop an immunity to this parasite, that immunity is specific to cryptocaryon and not generalized. However, it is also true that many fish will die without one of the three treatments. Diet and good water by itself is rolling the dice. it is your choice.
