Brad Pitt battles zombies for the United Nations in a film directed by Marc Forster. Illustration by Tomer Hanuka

After seeing “World War Z,” I walked through Times Square, and, as I made my way to the subway, I wondered why the movie—which is, after all, just a very expensive zombie flick—had excited and disturbed me so much. On Broadway, there were no zombies in immediate sight, only a happy and friendly crowd. Yet I felt a vague uneasiness at the sheer number of people milling about—a sense that they could all be transformed into something malevolent or frightened. Absurd? Mere paranoia? Maybe, but who hasn’t felt a tinge of paranoia while walking on crowded streets, particularly streets thronged with barkers for standup comics? The movie, which was directed by Marc Forster and was written by Matthew Michael Carnahan and a host of collaborators, evokes the hectic density of modern life; it stirs fears of plague and anarchy, and the feeling that everything is constantly accelerating. At times, it has the tone and the tempo of panic.

“World War Z” went through an enormous amount of rewriting and reshooting. Yet, despite some conventional passages and a soft ending, Forster and Brad Pitt, who is a producer of the film as well as its star, pulled the picture together. They also managed to reawaken in a large-scale movie the experience of shock, something that has been superseded, in recent years, by digital slam and much whooshing from nowhere to nowhere. I’m not an expert on horror movies, but I know when my heart rate has doubled. “World War Z” explodes right after the opening scenes, in which we see warnings that things are going awry: news reports of feral animals and dead dolphins, roving mobs and rioters—the usual premonitory noise that opens any disaster movie. Gerry Lane (Pitt), a retired United Nations trouble-shooter; his wife, Karin (Mireille Enos); and their two young daughters, driving in the family S.U.V., get stuck in traffic in downtown Philadelphia. Suddenly, the street erupts: motorcycle cops whizz by, a garbage truck plows through the waiting cars, there’s an explosion up ahead, and people start running, terrified. The speed and the violence hit you like lightning. Then, gradually, in brief, searing glimpses, you see them: human-looking creatures, with disintegrating flesh and wild eyes, biting and mauling people and causing general mayhem. These scenes suggest how quickly a major American city can fall into chaos. But what’s causing the behavior? A viral outbreak? At first, no one uses the word “zombie”—it seems too silly.

I was unnerved by the hurtling strangeness of the Philadelphia scenes, and I say this as someone who has sat through such war-of-the-worlds extravaganzas as “The Avengers” and the new Superman movie, “Man of Steel,” without feeling so much as a tingle. “Man of Steel,” which is more leaden than steely, is an example of everything that has gone wrong with so many big-budget spectacles. The characters declaim in blustering tones; ironclad bodies bash one another to pieces; there isn’t a joke or a satirical moment anywhere. The new Superman, Henry Cavill, bares his muscular chest; the hero is no longer a boy’s amiable fantasy of strength but the literal embodiment of power, with the heft and the thickness of a lifetime gym bunny. When war breaks out between Krypton and Earth, this physical paragon chooses our side. Manhattan (in the guise of Metropolis) gets flattened (don’t worry, Brooklyn is spared), yet no one you care about is in any danger. It’s an exhaustingly empty and emotion-free movie, a routine multi-hundred-million-dollar product.

“World War Z” is the most gratifying action spectacle in years, and one reason for its success is that Pitt doesn’t play a superhero. Gerry Lane is tough and infinitely resourceful (he could reset his own broken arm if he had to), but he doesn’t fly, he doesn’t pass through walls, and, in the long tradition of unpretentious movie heroes, he’s reluctant and terse. Early in the film, the Under-Secretary-General of the U.N. (Fana Mokoena), who seems to be one of the most powerful men left alive, tells Gerry that entire cities have fallen to the creatures, and he is the only one who can save the planet. So Gerry sets off in search of the first victim in order to find the cause of the outbreak—whatever it is—and then a cure. As he flies around the world, the audience is pulled slowly but ineluctably into the horror. At an American military post in South Korea, the soldiers know exactly what they’re dealing with. They call the creatures Zekes, and they would rather be shot than become one of them. The sequence is forbiddingly dark, rain-soaked, terror-streaked—a fleeting nightmare that recaps the pace and the charge of the Philadelphia scenes. You can admire a movie like Steven Soderbergh’s “Contagion” (2011), a realistic rendering of civil breakdown caused by a spreading pathogen, but the horror-film version of disaster in “World War Z” stretches the senses to take in more than you may expect. Pop dynamism can tear through normal expectations when the fantastic is as precise as this.

Vampirism, as everyone says, is about sex and violation. But what is the fascination with the hungry undead about? The origins of zombie lore in Africa and the Caribbean—voodoo, witch doctors, reanimated corpses—have long been appropriated by the modern media. The undead really do keep on coming; they are taking over our bookstores, our movie theatres, our cable channels. Every neighborhood has a zombie or two. Are they what we fear we might become if we let ourselves go—soulless vessels of pure appetite, both ravaged and ravaging? Do they represent our apprehension of what hostility lies behind all those blank faces in the office, at the mall, across the dinner table? In George Romero’s “Night of the Living Dead,” the low-budget classic from 1968, zombies tilt from side to side as they walk, slowly, their arms outstretched in an almost beseeching manner. Romero recognized our desire to understand, so the movie offers a mid-century sci-fi explanation: a space probe to Venus has returned home poisoned by radiation that infects unburied corpses. Despite this information, however, the movie remains unfathomable and tragic: a young girl, who has been bitten, attacks her mother. There was no safety anywhere. But that was just the beginning. By 2006, when Max Brooks published his novel “World War Z” (from which the movie is very loosely adapted), the worst has already happened: the zombies have been vanquished, but only after destroying a good bit of the world’s population. The survivors, in an “oral history,” recollect the past.

Zombies are so frightening, in part, because they prey on an infinite number of victims, and they don’t need weapons. They aren’t an army that can be licked; they have no purpose except replication. In the AMC series “The Walking Dead,” a group of human survivors band together and keep on the move, clearing safe havens for themselves. In their world, humans have been marginalized; the zombies have mostly taken over. The show devotes large chunks of time to the tangled emotional and sexual relations among the group, but it always comes back to a vicious turkey shoot, in which the women as well as the men attack dozens of the slow-moving undead, sticking sharpened poles into their faces, and slicing off their heads. In popular culture, apparently, anything can be justified by the need to survive, although some viewers may feel disgust at the righteous mayhem.