Ich (or at least the white spots that you see) are merely scars, as such, and not the parasite that infects the fish. In short, micro scopic parasites burrow into the fishes skin, there it grows until it is big enough to "split" into 100 so. As it burrows its way back out, the white spots appear.
They are like chicken pox - once the "pox" appear the infection is well set in, and curing the "pox" does not cure the infection.
Correct if I'm wrong here.
Matt