My view is that fish, depending on various stressors and levels of immunity (which can be lowered when stressors occur) may become less resistant to ich. That being said, I have always been able to "feed fish out of ich" using high protein, multiple feedings. I think that over time a tank will no longer contain ich as long as stability is maintained and the fish remain resistant. But many (maybe even most) people feed improperly and under those circumstances, coupled with the addition of new fish or other animals, ich can reoccur and evidence itself on fish that are less resistant or no longer resistant. I have read that resistance tends to diminish after 6 months but that is anecdotal.