Good question. I know Romero zombies were never really explained, although in " Night of the Living Dead ", it was briefly suggested that radiation from a space probe caused the dead to rise. In the " Return of the Living Dead ", it was the gas Trioxin that re-animated the dead. I've notice, ever since the debut of " Resident Evil", it seems that all Zombies now are a result of a virus,( which I don''t understand,how can a virus re-animate a corpse ? ). And by God, ever since Max Brooks stepped in to the picture with his " Zombie survival guide ", and " World War Z " books, and the Solanum virus, fanboys treat his zombie explanation as if it was Gospel.