To add to that, in some zombie stories, such as The Walking Dead, everyone turns into a zombie when they die, even if they haven't been bitten. I don't know if

To add to that, in some zombie stories, such as The Walking Dead, everyone turns into a zombie when they die, even if they haven't been bitten. I don't know if it's ever explained in the graphic novels, but in the TV show it's glossed over, with the character Rick Grimes saying something like "whatever it is, it's in all of us now". I forget if Romero's films share the same conceit. But in The Night of the Living Dead, the dead are actually coming to life, raising from graves.