Forster grew up in Switzerland, and as a child spent a lot of time looking at the anthills behind his house. ''I loved watching these insects crawl around,'' he says.

Later, he became ''obsessed with swarms of birds and fish, how these swarms communicate with each other''.

The fascination stayed with him when he became a filmmaker.

''I was always thinking I had to implement that image in a film one day, this swarm theory.

''And once I started working on World War Z, I felt there was no better way to implement that than in a swarm of zombies.''

One of the key set pieces in the big-budget apocalypse film, produced by and starring Brad Pitt as a United Nations worker trying to find patient zero in this unexplained plague, involves a swarm of zombies attempting to scale the impossibly high wall that's been erected around Jerusalem.

The symbolism of that wall is rich and conflicted: it protects Jews and Palestinians alike, in a brief and oddly blissful moment of coexistence now the threat has been transferred to an Other, more foreign and dangerous than either could ever be. But in Forster's mind the scene's significance is global, not local.

His zombies trample each other to (un)death in their frantic bid to climb ever higher. ''As the zombie tower is building it's almost like a feeding frenzy, them going after the last resources with no respect for each other,'' he says. ''They're basically this pack, crawling on top of each other.

''I thought it's a strong image because that's how humanity is.''

So the zombie apocalypse - the zombocalypse, as the kids would have it - in World War Z represents a planet on the brink of economic and environmental collapse?

''Absolutely, that reflects the time we live in … There are not enough resources for the people that exist.''

It's possible Forster is the first filmmaker to use the zombie horde in quite this way, but he has in no sense betrayed the genre by doing so. One of the key factors in the appeal of the zombie as a fictional device is that it is almost uniquely malleable as a symbol. It is what the linguists call ''overdetermined'', a sign whose meanings (the things it signifies) have overwhelmed the word itself (the signifier). It's a little like a zombie horde on the move, in fact.

''Zombies are a very elastic storytelling trope,'' the US novelist Jonathan Maberry, whose zombie trilogy (starting with Patient Zero) has been optioned for television, said in 2010. ''They're endlessly fascinating because they play on so many of our fears: the loss of identity in ourselves and our loved ones, paranoia, fear of disease, racism and so on.''

Above all, he added, they represent ''a constant and universal threat that is implacable and unbearable''.

The seeming endlessness of the onslaught is a crucial aspect of the zombocalypse scenario: the zombie horde represents a threat so enormous it seems impossible to defy. It is infinitely substitutable for the many things we suspect threaten us but we struggle to contemplate: to Forster's resource depletion and overpopulation we might add nuclear war, viral epidemics such as HIV and SARS, even mindless consumerism.

Each has generated its own iterations of the zombie story, or at least its own readings of the zombie genre. In the hands of left-leaning cultural critics, the film that started it all - George A. Romero's low-budget black-and-white Night of the Living Dead (1968) - was an allegory for the mindless waste of life that was the Vietnam War. His 1978 sequel Dawn of the Dead, with the action moved from a remote farmhouse to a suburban shopping mall, was a devastating critique of the empty lives of American consumers lulled into ignoring the ills of the world by the allure of all that shiny plastic.

The English zom-com Shaun of the Dead (2004) played a variation on the same tune: the repetitive pattern of life in suburban London is barely distinguishable from zombiedom; and in the end it's almost interchangeable, as Shaun and his best mate - by now a zombie, albeit one in restraints - resume the session on the gaming console they'd started before the plague set in.

Writing in the literary magazine Granta in 2011, Naomi Alderman, one of the developers of the app Zombies, Run, noted: ''The zombie apocalypse is the death of civilization. It represents the moment when all that becomes important is: do you have food? Do you have guns? We want to practice this in fantasy, to imagine it all the way through, especially in times of economic crisis.''

The ''philosophical counsellor'' Mark Dillof sees the genre as tapping into an anxiety about ''losing one's autonomy'', a kind of creeping ''paranoia'' - but one founded in genuine concerns.

All human societies are founded on borders, he argues, the most fundamental of which is that between the living and the dead.

''If the fear of zombies is prevalent today, it is because this is an age in which borders are being transgressed, on many fronts.''

Among them: non-traditional marriages, socialism, communism. ''Such movements as diversity, multiculturalism, globalisation, are threatening to dissolve the identity of various nations,'' he adds.

In the zombocalypse scenario, there is in fact precious little of the nation state left to dissolve. Arguably, this points to an anxiety about the rule of law in general and the viability (and reliability) of our political systems.

Nowhere is this played out more explicitly than in the US cable series The Walking Dead. The first two seasons of the phenomenally successful show (its season three finale was watched by 12.4 million people in the US in March) effectively pitted good cop Rick Grimes (Andrew Lincoln) against bad cop Shane Walsh (Jon Bernthal) in a world in which all systems had broken down. At every turn, they presented their small band of survivors with a choice - between the rule of law and the rule of the gun. In season three, the lines became more blurred as Rick sought to defend his tribe against a rival band of survivors led by an autocratic psychopath known as The Governor (David Morrissey) - and in the process became increasingly like him.

The show had struck such a nerve, season three show runner Glen Mazzara told me late last year, because ''people really do believe that some sort of apocalypse is coming and we are going to have to bond together to survive, and the infrastructure of society and government will collapse''.

''They imagine, 'What would my life be like if I had no food, no water and only a couple of guns and had to protect my family and my neighbours?'.''

If it taps into the fantasies of the US survivalist movement and its counterparts (of both reactionary and the greenie hue) elsewhere, the show also touches on a fundamental question about what kind of society would we choose if we were to start again. Would it be compassionate or would it be based purely on the survival of the fittest? For a US that often feels as if it has lost its way, it's a compelling premise. The zombocalypse scenario can satisfy far baser instincts too, as anyone who has ever played a first-person shooter game such as Resident Evil or Left for Dead (or the real-world version, IRL Shooter) will attest. In offering up an enemy that is both human and not, and presenting us with a simple kill or be killed scenario, it allows us to satisfy the killer instinct in a safe environment. Not for nothing was the virus in 28 Days Later called ''Rage'', even if the infected were never actually labelled zombies.

''Zombies are very simple, they don't have a complicated mythology like vampires or aliens,'' Mazzara says. ''They just keep coming at you - they're the never-ending presence of death.''

Writing in the Foreign Policy journal in 2010, Daniel Drezner, a professor of international politics, noted that ''zombie stories end one of two ways - the elimination/subjugation of all zombies, or the eradication of humanity from the face of the Earth''.

That is, in many ways, the choice many of us feel we face in the real world: we keep stumbling through, failing to act upon the threats we face, like so many zombies, or we rise up and reclaim our humanity, and our future.

''We're living in times where there are these really big problems,'' says Max Brooks, son of Mel and author of the book on which World War Z is based. ''We've got terrorism, economic problems, unpopular wars, social meltdowns … We're living in, not apocalyptic times, but I think we're living in fear of the apocalyptic times.''

Stories like his provide people with ''a sort of safe vessel for the end of the world,'' he adds. ''Zombies are safe. Zombies are manageable. You can't shoot the Gulf oil spill in the head.''

More than one fallout

Brad Pitt has a lot riding on World War Z. The big-budget zombie film is produced by his company, Plan B, in partnership with Paramount, so it's not just his face up there on screen - it's his dollars, too.

It's no secret that things didn't always run smoothly. Paramount bought the rights to the book on which it is based - written by Max Brooks, son of Mel - in June 2006 as a vehicle for Plan B. Filming was set to start in early 2009 but the project went back to the drawing board when the screenplay was rejected. By mid-2011 it was back on; a year later - as it ran over time and over budget and a new ending was ordered - Pitt and his Swiss-German director Marc Forster were no longer talking.

So goes the folk tale, at least. But according to Forster, who directed the James Bond film Quantum of Solace in 2008, much of what has been reported is simply not true.

''I finished the movie in the amount of days I was given, so we didn't go over schedule or anything like that,'' he says.

What of the rumours that he and Brad were not speaking? ''I was surprised when I read that. We were always in communication and respectful to each other. We never even had an argument or fight. I would say we are 99 per cent on board with the cut and the similar opinion. It wasn't like we had a communication breakdown.''

The concept of migration, and the experience of being a refugee, plays a surprisingly large role in the film. That ''reflects the time we live in,'' Forster says. But should we detect the hand of activist Brad - and maybe even Angelina Jolie - in that?

''Not at all,'' Forster insists. ''In fact, there was a concern to make sure it didn't reflect that they are so active in [that area]. It was actually me pushing a little more for that.''

However troubled its birth may or may not have been, the ending of World War Z seems very much to be aiming for a sequel. ''It's definitely in the air if it works out,'' Forster says. ''The idea we discussed originally was maybe a trilogy. But let's see how the box office goes and then we'll go from there.''

And if it is favourable, would he go there again, despite everything? ''It's always tricky with me,'' the director says. ''They offered me Skyfall as well and I didn't want to do another Bond movie. I like to switch genres. But in this case I definitely would leave it open. I don't want to close that door.''

Rise of the undead

Five of the zombie genre's finest instalments are:

White Zombie (1932)

Generally regarded as the first zombie feature, this black-and-white film stars Bela Lugosi as a Haitian voodoo master called Murder Legendre, who runs a sugar plantation staffed entirely by Haitian slaves. With its West Indian setting and focus on a magical explanation for the trancelike state of the undead (and lack of flesh-eating), it serves as a kind of goreless origin story for the modern zombie tale. Can be read as an allegory for fear of race taint; that might explain why it was one of the few Hollywood films the Nazis approved of.

Night of the Living Dead (1968)

George A. Romero's film cost $114,000 to make and has grossed an estimated $30 million, making it arguably the most profitable zombie film ever. More significantly, it is the film that kicked off the modern iteration of the zombie picture, with a small band of survivors under siege from a marauding pack of flesh-eating ghouls. Other key contributions: many of the main characters are killed off, the source of the plague remains unclear, and the leading man (Duane Jones) is black.

The Serpent and the Rainbow (1988)

Horror-meister Wes Craven plays it almost straight in this return to the Haitian setting of White Zombie, albeit while making a detour through the scientific-fringe of Ken Russell's Altered States (1980). Bill Pullman is a doctor on the trail of a drug that may have the power to bring people back from the dead; the voodoo aspect of Haitian culture is treated with something close to the seriousness of ethnographic documentary.

Shaun of the Dead (2004)

This English buddy movie-cum-romantic comedy - does that make it a Pom-zom-rom-com? - breathed new life into a by-then-old format, with Simon Pegg starring as a man stuck in a suburban rut that looked an awful lot like a living death. The pub as refuge/prison is a wonderful metaphor for … well, something.

The Walking Dead (2010-)

AMC's series, based on the comic books by Robert Kirkman, Tony Moore and Charlie Adlard, has taken the zombie genre to the masses, scoring audiences on cable television to almost equal any drama on American free-to-air. English actor Andrew Lincoln (he was Egg in This Life; he's Rick in this after-life drama) stars as the good cop determined to cling to some form of morality in a world gone to a very Darwinian form of hell.

World War Z is out on June 20.