vampy

Vampirism is a disease.  But unlike most diseases, this is not spread by a virus, bacterium, or parasite.  No, vampirism is spread via an errant pattern of magical force.  This magical force is capable of corrupting nearby magical forces.  Most of them are rendered useless or sent wild, making vampires highly resistent to magic cast upon them.  Souls, however, react differently.  The way that the soul is corrupted by the vampirism force pattern actually causes the soul to slowly rearrange itself into the pattern of vampirism.  This causes gradual physical and mental changes in the vampire.

The first change is that the vampire, due to losing portions of the soul, will feel compelled to eat more, as the soul attempts to sustain itself by absorbing lingering soul magic.  The closer the consumed soul magic is to the vampire's own soul magic, the better, and so meat and blood--especially meat and blood of the vampire's own species--are preferred.  However, vampires who haven't quite figured out what they actually crave yet will often simply eat whatever food is available in large amounts.  Consuming lots of soul power will somewhat stave off the progression of vampirism, but in the end it's just giving it more material to work with when it's time for the disease to spread.

The next change is physical.  After a few weeks, after the hunger has started but typically before the vampire resorts to full-blown bloodsucking, The soul has deteriorated enough that it has trouble sustaining the health of the body.  Pigments are the first thing to go.  The magical force of vampirism is enough to animate the body, so even as organs fail and the body breaks down, it will stay moving.  Eventually, unless they maintain their appearance, they will look like walking corpses.

The last change is mental.  Depending on the strength of one's will and soul, the mind may simply deteriorate and the vampire will descend into animal-level intelligence, seeking only to feed, or this change may be staved off for a while as the mind is corrupted into knowingly embracing vampirism.  However, no matter how strong one is, all vampires are doomed to one day lose their minds entirely.