Not sure if I have the answer - but do have some thoughts on this topic as I have advocated the "hair of the dog" approach many times in this forum. For starters, I do believe that fish can develop immunity to disease, including Cyrptocaryon. You essentially have to ask yourself if you believe in vaccines. If you don't - do you vaccinate your kids? And if you do vaccinate your kids, why? The same general principles apply to both mammalian and piscine immune development, although we know significantly more about the mammalian immune system.
There are many vaccines that have been developed for fish that work and have been shown both in the lab and field to provide good long-term protection. These include live vaccines that are very potent in eliciting an acquired immune response. This is key since there are many arms to the immune system - and what we are looking for is activating an acquired response with good, long-term memory. In the case of Ich (mainly based on the freshwater model) we know that the protozoan's cilia confer certain antigenic determinates - that is to say they stimulate the immune system. However, characterising the response by vaccinated hosts has a long way to go. In most instances, vaccine preparations (which contain killed cells or purified cilia/cilia proteins) try to replicate an infection - without causing pathology. However, such preparations are very different from live infections since they are "seen" differently by the immune system. It is therefore reasonable to assume that if a fish survives a mild infection then this would essentially act as a live vaccine.
The tricky part in all this is that controlling such infections can very difficult. But I think it can be done. It all boils down to environmental factors - the physiological state of fish (ie level of stress) and the environment it is living in. Some environments are inherently more stressful than others (eg. poor water quality, aggressive tank mates etc). This is why leaving some fish to 'fight off' an infection seems to work for some and not others. I think if your fish are healthy and stress free - they stand a good chance of fighting off the infection and developing immunity. Thus, my approach has been to provide an environment that fosters immune development and only step in with chemical intervention when the infection appears to be running out of control. As a result I have seen Cryptocaryon several times, but have never lost a fish to it, or had to treat with either hyposalinity or copper. However, many would advocate that treatment with hypo or copper are the only effective ways to deal with the parasite and not dealing with it means your fish are guaranteed to die. Simply not true in my opinion. It really depends on the specific situation.
The comment about strains is a very important one. Since there are often many strains of pathogens where immunity is not always cross-protective. This effect is seen in many infectious organisms and as a result many fish bacterins (vaccines against bacterial disease) often need to include multiple strains (basically to hit the optimal mix). Sometimes this works - sometimes it doesnââ"šÂ¬Ã¢"žÂ¢t. Streptococcus iniae, which commonly infects warm water fish, is one example of bug that is notorious for being very strain specific making commercial vaccine development difficult. Further, and this is very important where strains (or different serotypes) are concerned, some are inherently more pathogenic than others. Even more important, many strains can often attenuate over time - that is become less pathogenic. (The reverse can also happen - but usually only when artificially propagated from dead hosts). When a new fish is introduced two things can happen - an old strain that your fish are immune to, but the newcomer is not, can become problematic in the new fish. Or, the new fish may introduce a new strain to which your fish are not immune. For this reason quarantine is a valuable tool - but is not 100% foolproof. It is also why there are so many different experiences on the subject of Ich and Ich control.
This brings me to my last comment and that is infectious load. While I think it is possible to manage Ich by allowing fish to develop immunity - it really boils down to numbers. If the infectious load is too great the immune system will simply not be able to deal with it. Further, if your fish are heavily stressed this could suppress the immune system allowing a small infection to be a clinical mess. So if your fish are very stressed they won't have a chance and a large outbreak will ensue. If present in the same tank "immune" fish could very well become infected and succumb. Having said that, just because you quarantine your fish doesn't mean you will avoid Ich. It just means you reduce your chances significantly. Even if you treat your fish with hypo or copper during quarantine there is no guarantee you've eliminated it. It will, however, reduce the odds of seeing the parasite significantly further. Keep in mind that just because you don't see a spot on visual examination doesn't mean it isn't there. On the other hand, such treatments and practices must be balanced against the odds of stressing and potentially killing your fish with the treatment - which many experience. Even if you do eliminate Crytopcaryon from your fish - if you water quality goes off and your fish get stressed (for whatever reason) they will not do well and there is still a good chance that they will become infected with something else (unless of course you believe your fish and water are sterile...).
For me, the bottom line is to use quarantine as much as possible but to recognise that it is not foolproof and can be risky for very sensitive fish, but to also recognise that with good husbandry fish can develop immunity to many (but not all) pathogens. I therefore focus on fish and the environment - rather than the pathogen.
