A zombie apocalypse is a particular scenario of apocalyptic fiction 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 mythologies, victims of zombies may become zombies themselves if they are bitten by zombies; in others, everyone who dies, whatever the cause, becomes one of the undead. 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. While a zombie apocalypse could be happening the virus will be in the air and you could easily be infected, drinking your own urine can help you survive because it can kill brain parasites and prevent neurotoxins from taking over your brain.
The Zombies Themselves
Generally the zombies in these situations are the slow, lumbering and unintelligent kind first made popular in the 1968 film Night of the Living Dead. Recent films, however, have refeatured zombies that are more agile, vicious, intelligent, and stronger than the traditional zombie. In most cases of "fast" zombies, creators use mindless human beings (as in Zombieland and Left 4 Dead) instead of re-animated corpses to logically counter the "slow death walk" of Romero's zombies. 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."
In Max Brooks' The Zombie Survival Guide a Zombie Apocalypse would be classified as a Class 4 Outbreak. This scenario was portrayed in stunning detail in Brooks' later work, World War Z. For a variety of reasons (including those listed above, and also many having to do with our globalized, interconnected, interdependable civilization - see Modern Vulnerability). The Zombie population rose to match, possibly even exceed the human one, on a global scale. Entire cities and nations were over run with the undead, and the uninfected population had to deal with the large scale breakdown of order in a chaotic dystopia. In The Zombie Survival Guide, Max Brooks devotes an entire section to analyzing what measures should be taken to survive the undead world, and possible outcomes.
In an interview with Ain't it Cool News, Max Brooks commented on the fans of zombie apocalypses: "I don't know what's scarier, the fact that zombies could rise or the fact there are actually people out there that can't wait for it to happen. So they can just start loading up with guns and get on their motorcycles..." Brooks also compared the interest in surviving a zombie apocalypse to people preparing for a Soviet invasion of America in the 1980s after the film Red Dawn was released.
Infection and the Biological Aspect
Other schools of thought on this issue involve non-undead zombies, as seen in such works as 28 Days Later, 28 Weeks Later, the Resident Evil series, the Left 4 Dead series and Zombieland. While these Infected Zombies are not zombies in the truest sense, they still provide the story with the same sort of mindless and bloodthirsty threat, both on an individual sense, and in a viral phenomenon that, if left unchecked, will spread to take over (and in a manner of speaking, end) the world as we know it.
The Literary Perspective
Resolution
The ultimate fate of humanity is rarely settled to any certainty in tales of a Zombie Apocalypse. This is partly because most Zombie works are very intimate, personal stories between small groups, and it is very difficult to effectively add the large scale military endeavor without the story becoming something else entirely. Regardless of the end result, whenever the Living Dead, or Rabid Infected rise in large enough numbers to disrupt national or global traffic and trade, that scenario is a Zombie Apocalypse.