Vampires. Specifically, vampires in the Buffyverse.
So, first let’s back it up. Origin stories vary, but in the Western version of the vampire, the one most commonly associated with Dracula-like vampires, the ones whose main aversions include holy water, silver, and the Cross, the ones the Buffyverse vamps are based on, it pretty much always has to do with a curse from God.
One version is that they come from Cain (sexy, sexy Cain.... hm? wha? sorry, i got distracted) and are cursed by God because he was the first murderer, so holy water aversion makes sense, as does the Cross, as Jesus’ sacrifice is supposed to be, in part, caused by Cain’s actions. Silver makes less sense, except in that silver is considered “pure” and vampires are “impure.” It’s the same reason silver works on werewolves. (Granted, wouldn’t gold make more sense? Silver tarnishes, but gold doesn’t? Whatever.)
OK, second version of vampire origin is that Judas was so cursed by God for his betrayal of Jesus that when he committed suicide he wasn’t allowed to die and became the first vampire. Again, the aversion to “holy” things makes perfect sense here, especially aversion to the cross, and even the silver has special significance since Judas betrayed Jesus for 30 pieces of silver.
So far, so much makes some sort of sense. (The whole idea that God would curse someone in this way, especially a “curse” that grants immortality and power and can be passed along to innocents makes much less sense, but ¯\_(ツ)_/¯ ) The traditional Dracula version is pretty similar in theme, just with a more obscure (unless you were from 1400s Romania) dude. Vlad was really bad and rejected God and got cursed for it. So, now he can’t come near the signs of Christ’s redemption, ie, things like holy water and the Cross, etc. Again, the silver thing makes a bit less sense, but, purity/impurity, go with it. Stake through the heart to kill it starts to make real sense with ol’ Vlad, what with all the impaling and what-not.
Since the vampire mythos springs from many cultures all over the world, and has crossed and re-crossed and different versions of the stories have influenced each other, it’s hard to come up with any one “origin” for the vampire. Also, it’s difficult to parse out what abilities/weaknesses each version of the myth would have had since again, they tend to cross over and pollute each other. The silver thing may have started with the Judas version, but moved over into the Cain and Vlad versions because reasons. Lather, rinse, repeat with the stake through the heart, but start with Vlad and work into the others. It doesn’t really matter here, because, again, they all have in common the curse from God and thus the aversion to Christian religious iconography.
So, enter my favorite vampires. Whedon vampires. They seem to be based, at least in their abilities and the stuff that kills them, mostly on these traditional views of the vampire that are held in Western culture (specifically English/American) and that get solidified by Bram Stoker. No turning into mists or bats (expect for when Buffy really does meet Dracula) but in general that’s what we’ve got. They have aversions to holy water and the Cross, they get killed by a stake through the heart, etc.
Of course, the great thing about creating your own version of a classic monster is that you get to give it your own backstory. In Buffyverse lore, we know that vampires came into being when pure demons mixed their blood with the newly-arrived-on-the-scene humans. Thus, vampires are a demon/human hybrid. The demon parts in question predate humans by untold millennia. In the Buffyverse, no other demons seem to care in the slightest, but vampires still have an aversion to those traditional “anti-vampire” measures: holy water and crosses. Why? They.... shouldn’t. In this version of the mythos, they haven’t been “cursed by God.” They haven’t rejected God, or Christ, or whatever. (They have, possibly, rejected puppies, since Spike does refuse to eat that one puppy that one time, but that’s hardly a mortal sin.) And, in fact, they have been around for thousands of years before there was a Jesus to be martyred on the Cross at all, or priests to pray over water and bless it. Did pre-Christian slayers have nothing but stakes to rely on? Did other religious iconography work, BC? A nice ankh, possibly? A faravahar? Maybe Willow didn’t have to nail crosses to her wall to revoke Angel’s invitation after all. Maybe she could have gone with a mezuzah.
In conclusion, I find this lack of in-universe consistency distracting. Especially when so much other stuff is so very consistent and well thought-out. Except for that whole “let’s curse him by giving him back his soul, so that he’s all tormented and guilt-y, but we’ll just leave in this little loophole that if he ever has even one little moment of non-torment, curse lifted, soul goes away, evil monster is free to kill with impunity, no more punishment” thing.
I mean, really. Not a well thought-out curse, people. Do better next time your princess gets brutally murdered, would ya?