Creatures of horror films are transgressive by nature. Monsters are monsters because they violate human categorical boundaries. Mummies, zombies, vampires et al are impure because they are “categorically interstitial, categorically contradictory, incomplete and formless”*.

Now, the fact that monsters contradict natural categories doesn’t mean that their nature has to be contradictory. “Even the undead ought to obey a few simple rules”, says Stephen Harrigan who complains about there being many inconsistencies among celluloid vampires. 

 First, it is not clear how vampires are killed. In Murnau’s Nosferatu, a vampire dies “if a woman ‘pure in heart’” lies beside him all night “until ‘the cock has crowed’", while in 1931’s Dracula “Bela Lugosi has to be impaled through the heart in his coffin”. But then, Tom Cruise in Interview with a vampire “informs us that the stake-to-the-heart method is ‘nonsense’” and Blade and Carpenter’s Vampires provide their own recipes to kill such bloodsucking creatures.

The case of transmogrification, as Harrigan calls it, is no different; nor is the question of motility. If you think someone turns into a vampire just by being bitten by one of them, you’re missing the subtleties of Interview with a Vampire or forgetting that in Blade, as in “several other AIDS-conscious vampire movies” vampirism is “an infectious blood disease”. Similarly, if you assume that vampires can fly, it is because it’s recent films what you have in mind; 1921’s Nosferatu, in turn, “shuffles arthritically around his castle, and when he rises from his coffin he's as stiff as an ironing board”. But not even something as salient in vampires as dentition is, has a standard: it is not clear whether fangs are retractile or fixed. Actually, Harrigan reminds us, in the case of Bela Lugosi, we never get to see his canine teeth.

To mitigate such a lack of canon of vampirism, Harrigan proposes some standardizing criteria. Here’s a summary:

Mortality and Mortification: “crucifixes and garlic to be regarded as nonlethal irritants. Vampire death to be assured by penetration of heart muscle by any foreign object or by prolonged exposure to sunlight”.
Transmogrification: “Vampirization to be contingent upon total extraction of victim's own blood and its subsequent replacement by blood of donor vampire. If more than 24 hours occur between initial suckage and revivification, victim no longer qualifies for living death designation and will be considered conventionally deceased.”
Motility: vertical flight only, to a maximum of 20 feet above the ground. Sustained flight permissible if vampire takes the form of a bat, owl, or other authentic nocturnal species. Vampire is specifically prohibited from turning into a flying homonculus.
Dentition: “retractable fangs as default characteristic. Strongly recommend, when appropriate, on-screen discussion of physical requirements for said phenomenon--as when a character in Blade observes an "odd muscle structure around the canines."

It’s always good to know what a canonic vampire is like. But just in case, never trust someone who wants to bite your neck.

More on this at slate.com