There was no telling how to cure a sentient disease, nor a way to separate host and symbiote safely. Unlike Othre's situation, there was no vortex messing with the healers' mana, but the entity didn't reside in a specific part of their victim's body.
It was so widely spread that removing it surgically as Manohar did with Thrud's meat puppets was impossible.
"Isn't it obvious? The source of the disease is a plant, and undead or not, Fae don't leave a body behind. Hence, if we find the one whose tissues were used and kill them, all of our problems will be solved." Milea said.
"Another possible option is to identify the undead who spawned the parasite. It will not only help us to narrow our search, but it could also help us in finding a way to neutralize the affliction.
"Aside from Liches, all other undead have several weak points. This isn't a disease so much as a living being, which means that it must share the flaws of its creator." Marth said.