A zombie apocalypse is a particular hypothetical scenario of apocalyptic theory that customarily has a science fiction/horror rationale. In a zombie apocalypse, a widespread rise of zombies hostile to human life engages in a general assault on civilization.
In some stories, victims of zombies may become zombies themselves if they are bitten by zombies or the virus travels by air; in others, everyone who dies, whatever the cause, becomes one of the undead. In the latter scenario zombies also prey on the living and their bite causes an infection that kills. In either scenario, this causes the outbreak to become an exponentially growing crisis: the spreading "zombie plague" swamps normal military and law enforcement organizations, leading to the panicked collapse of civilian society until only isolated pockets of survivors remain, scavenging for food and supplies in a world reduced to a pre-industrial hostile wilderness.
The day that the zombie apocalypse begins is sometimes known as Z-Day, by analogy with military operations such as D-Day.
The founding work of the genre was Richard Matheson's novel I Am Legend (1954), which featured a lone survivor named Robert Neville waging a war against a human population transformed into vampires. George A. Romero borrowed the idea for his apocalyptic feature Night of the Living Dead (1968) but substituted vampires with shuffling zombies
The literary subtext of a zombie apocalypse is usually that civilization is inherently fragile in the face of truly unprecedented threats and that most individuals cannot be relied upon to support the greater good if the personal cost becomes too high. The narrative of a zombie apocalypse carries strong connections to the turbulent social landscape of the United States in the 1960s when the originator of this genre, the film Night of the Living Dead, was first created. Many also feel that zombies allow people to deal with their own anxiety about the end of the world. Kim Paffenroth notes that "more than any other monster, zombies are fully and literally apocalyptic ... they signal the end of the world as we have known it.
According to a 2009 Carleton University and University of Ottawa epidemiological analysis, an outbreak of even Living Dead's slow zombies "is likely to lead to the collapse of civilization, unless it is dealt with quickly." Based on their mathematical modelling, the authors concluded that offensive strategies were much more reliable than quarantine strategies, due to various risks that can compromise a quarantine. They also found that discovering a cure would merely leave a few humans alive, since this would do little to slow the infection rate.
On a longer time scale, the researchers found that all humans end up turned or dead. This is because the main epidemiological risk of zombies, besides the difficulties of neutralizing them, is that their population just keeps increasing; generations of humans merely 'surviving' still have a tendency to feed zombie populations, resulting in gross outnumbering. The researchers explain that their methods of modelling may be applicable to the spread of political views or diseases with dormant infection.