For humans and animals, the best way to make more humans or animals is through sex, or, if you’re really kinky, through cloning. For the undead creatures of horror movies and fiction, the best way to reproduce is through biting. If a zombie bites someone, it makes a another zombie. If a vampire bites someone, you get another a vampire. If a werewolf bites someone, you’ve got a brand new werewolf.
A question that’s been on my mind recently (if recently means the past 16 years) is what happens if a human is bitten by a vampire, a werewolf, and a zombie all at the same time. I tried asking this on Metafilter, and a rousing discussion began but the topic was closed by a moderator before it could really get underway (you can see the deleted thread here.) Since that thread was closed, I’d like to see if I can renew the discussion here, because (I thought) it was fun and interesting.
I know there’s lots of variables to consider, because depending on which movie you watch or which book you read, the undead can follow different rules. There’s also the question of how this could possibly happen in the first place.