Vampires are born out of a fundamental change which can occur in the physical bodies of creatures which possess a soul. In most mortals the soul is the fundamental source for action, creating the original impulses which eventually become the person's acts. It is also a passage through which the creature can gain a small amount of the infinite power underlying the universe, power which is necessary for the functioning of a self conscious being.
A vampire is born when a mortal creature undergoes a change which causes the sensual physical body to revolt against the soul, usually expelling it, and taking control of the creature. The actions of the being are then controlled by either the remaining physical brain, the remnant of the soul, whatever caused the expelling of the soul, or a conglomerate of these forces. It is this emphasis on the physical body to the exclusion of the soul, which causes many vampires to be obsessively sensual and addicted to their own suddenly intense perceptions.
There are many things which can cause a creature to become a vampire. These include certain forms of intimate demonic possession, various viruses, the casting of spells to become a vampire, or mental disease. Each of these causes has its own peculiar nature, and can result in very different outcomes for the victim.
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