No reason is given in Shaun of the Dead for the zombie invasion. A news reporter tells the citizens that the only way to kill zombies is to remove their heads or destroy their brains. In this extremely bloody and gratuitously violent flick, heads are bashed until they fall off, bodies are impaled with whatever metal object is available, and zombies dismember and hungrily eat human flesh. It’s grizzly. If the producers of Shaun of the Dead removed all of the filthy language, blood and gore, all that would be left is Shaun and Ed sitting on the couch like zombies playing video games.