Since at least George Romero's 1968 Night of the Living Dead, which redefined the concept of the zombie mythos to what most Americans know today, the theme of a "zombie apocalypse", in which an ever-growing horde of mindless walking corpses who spread their contagion by feeding on the helpless human population inexorably overrun the neighborhood, city, country, and even world, has remained extraordinarily popular in the mainstream imagination. This sub-genre of horror film provides a particularly rich, if mostly unrecognized, commentary on mass society. For example, recent genre offerings have tended to focus on the spread of a zombie contagion in largely urban areas, where the infection spreads quickly and easily. The dwindling number of survivors increasingly have nowhere to run as the zombies seem to be everywhere. Zombie apocalypse stories continually reprise several notable themes: fear of pandemic contagion (sometimes originating in shady bio-engineering research), ineffectual governments, survivalism, breakdown in social order, and so on. However, it is the ability of the zombie movie to tap into the deep-seated but largely unconscious tension inherent in living within mass society and particularly in the ambivalent confines of the modern urban landscape that represents the genre's greatest subversive potential.
Like other primates, humans have evolved over several million years to spend virtually all their time together with their tribe or band. I think it is fairly obvious that a lone human being, no matter how fit, has little hope of surviving alone in a wild environment. Band society, characterized by cooperation and sharing, enabled humans to live, and live well, for millions of years. Simply from a pragmatic viewpoint, in-group betrayals such as murder, lying, stealing, and so on would have been intrinsically discouraged (though not inconceivable) due to the inherent lack of "hiding places" in such an intimate social environment--in other words, there would have been no wall of anonymity behind which to escape after committing a transgression in a small group of people wherein each member depends on the support of the others, not merely for survival, but company, entertainment, affection, a sense of belonging, and the psychological benefits thence derived. Additionally, I'm hard-pressed to think of very many things in a nomadic band society that could actually be stolen or lied about. In any case, the consequences of unacceptable behavior are immediate and obvious in a closed band society, even if the punishment is simply being ostracized or ignored. Strangers, by contrast, offer none of the aforementioned benefits to a group and would indeed be able to get away with just about any act of deceit or aggression as long as they were able to escape before they were caught. The consequences of harmful behavior virtually disappear once the offending stranger escapes--if the stranger escapes. However, without the presence of other members of one's band, one becomes vulnerable, and there is virtually nothing stopping a stranger from harming you.
Past a certain age, children stop acquiring language at their peak rate--apparently, nature did not deem it worthwhile to prepare us for the possibility of traveling abroad and having to acquire a new language at 25 years of age. At the genetic level, we are all ill-disposed to mass society and engaging with unfamiliar factors. Around the same age that they stop learning language efficiently, children also tend to become wary of strangers. This is not to say that children don't become braver and more willing to explore their surroundings away from the protection of adults, but this wariness of encountering strangers, especially unexpectedly, never disappears, even despite our modern liberal enthusiasm for embracing globalization and reveling in the melting pots that are supposed to be our cities. How can one tell if a stranger means us well or harm? What if the supposedly peaceful inhabitants of our cities turn out to be malevolent? They already surround us, potential threats literally occupying all the space above (skyscrapers), below (subway system), and around us. Through lack of choice, we have adapted to these novel environments, but only imperfectly, as the anxiety and alarm that the fundamental distrust we feel toward the strangers occupying our surroundings evokes must constantly be suppressed in order to function in a city. Unable to affect our own situations, like chickens in our crowded, putrid, windowless sheds, we routinely ignore the motley assortment of passengers sharing our bus or train car. We expect rudeness and perhaps even a degree of danger in some parts of our cities as a given. Some individuals seem unable to cope well enough to function in this environment, and we may term these people agoraphobic, anxious, neurotic, paranoid, or delusional. Still, even those deemed "normal" in the city seem unable to function unless they cloister themselves away from the madness that threatens to encroach from all sides, whether they lock their eyes onto the screens of their smartphones and deafen their ears with streaming music through their ear buds to get through their daily commute without having to interact with unfamiliar people or withdraw into a sudoku puzzle or the day's New York Times at a coffee shop full of obnoxious patrons who, despite your best efforts to isolate yourself, may yet succeed in destroying your solitude through various means, such as the indefensible assault on your nostrils from an obnoxious perfume or a too-loud couple conversing just above your head. Zombie films may be interpreted as a distillation of these suppressed anxieties writ large on the silver screen, like a nightmare derived from our habitually-suppressed stress about strangers in our environment that we can safely (because it's "just a movie") experience consciously but whose source from within our own psyches still remains mostly obscure to the waking mind. Nevertheless, if something of the recurring motif of zombie ascendancy did not resonate deeply with people, we would be hard-pressed to account for the genre's immense popularity.