The Walking Dead takes place after the onset of a worldwide zombie apocalypse. The zombies, referred to as "walkers", shamble towards living humans and other creatures to eat them. They are attracted to noise and to scents, including the scent of humans. Humans who are bitten or scratched by walkers die and become walkers themselves. Early in the series, it is suggested that, because any human who dies will reanimate as a walker regardless of cause of death, all living humans carry a pathogen responsible for the mutation that turns humans into walkers. No pathogen is ever confirmed, however. The mutation is activated after the death of the host. The only way to permanently kill a walker is to damage its brain or to destroy the body entirely