Without knowing how you've TRIED to cure it, I can't tell you why you can't cure it.
Throwing sage in the tank and chanting, "Huwear shor-shortz, we-wear shor-shortz" won't help you. :-D
The thing about parasitic infections, though, is that most parasites don't want to negatively effect the host -- not much anyhow. If they outright killed it, they'd die with it. Their best strategy for long term survival is to have as little impact as possible. In short (heh), fish can live a long, successful life with parasites -- just like you could have tapeworms for years and be completely unaware. Then, something happens to knock the animal out of balance, and the parasites over multiply and ... you have the above situation.
Have you tried treating with Prazi Q?
SOMEONE still has my big bottle of it, but if that SOMEONE returns it anytime soon ... I'll hook you up with a dose of it. How big is your tank?