Have you ever touched a dead body. Cold Brrr! I have to laugh when I hear people talk about how “hot” that vampire is… In my universe, as always, there has to be some understandable reason as to why dead flesh should reanimate. For me, there are several ways in which that can happen.
In ancient days, the gods were responsible for keeping the spirits of the dead until the time of dissolution. For the gods, it was an easy thing to reanimate flesh and blood. As Doctor Who once quipped, (for the gods) “life is just nature’s way of keeping meat fresh.” A simple decision on the part of a god could bring the dead back out of the ground. They could be cleaned up, as needed, and endowed with a task or whatever. With a special dispensation from the King of the gods (or community of gods) the spirit of that dead person could re-inhabit the flesh; though if the idea is to bring a dead person back to life it would be simpler to simply re-clothe the dead spirit with a freshly made flesh and blood form.
In my universe, there is almost nothing a god cannot do, but for the rest of the universe, such a thing might not be so easy.
Magic can certainly be used to revive the dead, but it is not an easy thing to do and there is a risk in doing so. Science, as in the virus from “Night of the Living Dead” or as in Doctor Frankenstein’s monster, can do much the same. But the risk remains.
That risk is found in the spiritual realm where the demons cast from Heaven remain disembodied spirits not permitted to take on flesh and blood. They can certainly possess a living person if invited in, but otherwise they look for just such an opportunity. A reanimated dead body just calls for inhabitation, and then the magician had better beware, big time.
Zombies are the reanimated dead. Even in such a state, they have a limited lifespan. For one, they are still in a state of decay, so it will only be a matter of time before the form becomes untenable. They avoid fire, certainly, but also running water or anything else that might hasten the decay process. A demon inhabited zombie can respond and relate to people and situations to the extent that the brain remains more or less intact. This is why zombies are often attracted to people and places they were attracted to in life.
Vampires, on the other hand, might be called the reanimated undead. Generally, vampires are brain-dead, as a doctor might describe it, but not exactly “body” dead (yet). In that case, a demon can get long life out of a form. As long as the heart is still pumping, even if all the other organs have stopped functioning, the demon can live.
Of course, with the reanimated brain once again getting the blood it needs, memories of that person, personality traits, even conversation is possible. Thus a vampire (unlike a zombie) can appear as a normal, functioning person (as long as it is in the dark – demons instinctively avoid the light). They can appear like the person even to the point of fooling those familiar with that person that they are that person… It’s all in their head.
Now, with the organs not functioning correctly, including the spinal cord, there is a need for fresh blood, human blood to keep the body cells from deteriorating. Live forever? No, but for longer than a normal life? Certainly. Of course, a stake through the heart… No, not a steak dinner and a movie…