This really seems a bit overpowered to me, even for a 9th level spell. Sounds more like an epic spell to me. If I was going to weaken it to make it more like a 9th level spell, I would say that the entire curse ends 1 day/level after it was cast, after that infecting no more people but continuing to cause damage to the ones already infected. Just my humble opinion though.
-Xeran