Hello, my dearlings; I do believe I've found something. The Council is readying the war drums, but in the meanwhile I thought it best for us to confabulate on the matter.
I was in the library digging up anything on mystical toxins from the days of yore, and -- I kid you not -- I managed to find a dusty, rotten first edition of the Mortissima by Amarinta von Hapsfeldt, a very rare volume you no doubt read excerpts from in your white magick classes. I didn't even know we had a copy in the tower! It must be worth a fortune, even in such a dilapidated state.
The original printing has all of her case studies and appendices, things I've never seen before -- and there's a very interesting account that reads like what we're experiencing here. The long and the short of it is that while I haven't identified the illness per se, Lady Amarinta speaks of an herb she calls the 'all-salve'. Whatever it is, it grows only in a very specific region of Ivalice, and it's guarded by some kind of horrible monster. Isn't that always the way? But if we defeat the beast and bring the all-salve home, we may be able to do something about this plague.