There was eventually going to be a cure. If it were only Muirne looking for the cure, it'd take A LONG, LONG time before it was ever found (mainly because she's only one woman having to deal with a LOT of sick people at once...). So far as it affecting all schools of magic, excepting immortal forces, I can see that being simpler. But, perhaps like I stated earlier, it would only affect humans and other human-esque beings, such as elves and dwarves (I'm doing this primarily because I wouldn't know how the immune system of something like a dragon or a deer-woman would work, although in either case, I can see the disease being a minor inconvenience for them rather than a life threatening problem).
Also, I got to thinking about factions and whatnot. Obviously, you'd have people trying to cure the disease and help those infected (Muirne definitely falls into this group), but I also think there'd be some superstitious or religiously zealous people trying to appease the gods to lift the plague? Ideas on that, guys and gals?