Ich is relatively "mild" as parasites go. Some fish will show symptoms, some will not. The immune system may adapt well enough to manage the presence of the parasite or even build up immunity.
However, so long as the parasite is lurking in your tank the potential for disaster always exists. I've lived most of my life in South Louisiana, and was able to "manage" ich being in my DTs for years & years. And then the hurricane would come, my tanks would lose power, temperature would rise, and suddenly all the fish would break out & die. It's these "stressor events" that occur which remind us we should have just taken care of the problem long before it turned into a catastrophe. o_O
