Corpse Plague manifests when a large group of souls don't pass on to the divine realm, and their presence begins weakening the veil. With a weakened veil they are able to possess corpses, which causes a reaction in the physical body prompting the Corpse Plague to form, the vile sickness can then quickly spread to living bodies, those of weak will are easily turned. When the outbreak reaches its peak in a concentrated area the veil can weaken to the point a demon can manifest in the physical world.