Two weeks ago I wrote a list of five doomsday films ranked by plausibility. The response to this piece made me realize that I overlooked the most pressing apocalyptic threat of all: zombies.

The onslaught of the living dead has been a mainstay of horror cinema for decades, beginning with the Bela Lugosi vehicle White Zombie in 1932. Over the following years zombies popped up in movies as one of many monstrous villains, often filling the minion role. It wasn’t until George Romero’s groundbreaking 1968 film Night of the Living Dead that the idea of a zombie apocalypse was introduced. Romero’s cannibalistic zombies have since become the archetype used in countless films, books, and video games. The cause of the virulent plague of the walking dead varies, however. Everything from spiritual curses, viruses, chemical weapons, and alien microorganisms have been used to explain the origin of zombies. Below the jump we examine the real-world evidence behind some of these threats, and which ones you should be most worried about.

Much of the popular lore surrounding zombies can be traced back to Haitian Voodoo. Sorcerers, known as bokors, are said to be able to animate the dead, forcing them to work as their slaves or warriors. This is more or less what Bela Lugosi was up to in White Zombie, except that he was a wealthy white plantation owner who used zombie slaves to work his sugar mill. It was all just business until he tried to use his power to improve his ailing love life; then things got wacky.

In the 1980s, Harvard ethnobiologist Wade Davis went to Haiti to try to track down a physiological basis for Voodoo zombification. He discovered that the bokors used a special “zombie powder” to induce trances in victims, which could apparently last for years. The powder contains a poison known as tetrodotoxin, or TTX. Davis wrote a best-selling book about his research adventure, The Serpent and the Rainbow. The problem with Davis’ theory is that the powder, while it does contain decaying human flesh and God knows what else, has only a very small amount of tetrodotoxin. Even if it did contain more, it would only slur your speech, stifle your breathing, and possibly kill you, but it wouldn’t turn you into a sugar plantation zombie slave. If the bokors get technical and decide to weaponize zombie powder, then we could have a real apocalyptic threat on our hands. But it would be more like an old-fashioned chemical weapons attack, not a true zombie plague.

For the walking dead to go from being a mere nuisance to a force capable of ending humanity, the affliction really needs to be contagious. In Night of the Living Dead, those killed by zombies soon rise to join their ranks, thereby growing the undead horde at an exponential rate. This model has been used in countless films since, with varying explanations about the nature of the zombie plague.

In Romero’s classic, a reporter says something about a probe returning from Venus having exploded in Earth’s atmosphere. While it’s possible to imagine that a virus from an alien world could do just about anything, including raising the dead, other zombie movies have searched for terrestrial origins for zombification. One idea is that rather than zombies being reanimated corpses, they’re living people afflicted by a really nasty form of rabies. I Am Legend and 28 Days Later can both be read in this way. While the rabies model does account for altered behavior, and transmission through biting, there’s something about the voracity and speed of a rabid being that just isn’t quite zombie-like. The afflicted hordes in both films move much faster than Romero’s zombies, which would serve to spread the infection at an accelerated rate. As much as I love 28 Days Later, slow moving zombies are scarier, even if they’re a little easier to fend off. Either way, if scientists were ever to alter the rabies virus to have an extremely short incubation period and heightened symptoms, we could have a whole lot of very crazy people with the ability to infect more.

The Return of the Living Dead, released in 1985, added a key element to zombie mythology: brains. Where Romero’s ghouls wanted to eat all human flesh, Dan O’Bannon’s zombies hungered specifically for human brains. In Return the cause of zombification was a bungled military experiment involving a poison called trioxin, which is a real chemical, but thankfully it does not cause people to become nearly indestructible walking corpses. Eating the brains of the dead, however, can have some horrifying real-world consequences. In the 1950s an American physician and researcher discovered a troubling disease affecting the Fore tribe of Papua New Guinea. They called it kuru, which means “trembling with fear.” The neurological disease caused patients to shake uncontrollably and burst out in maniacal laughter before dropping dead. On a cellular level, the condition is not unlike Mad Cow Disease, and if you know anything about how that is spread, you probably know where this is going. It turns out the Fore people had rather strange funeral practices, which involved eating the bodies of the deceased. Kuru was more prevalent among women and children, because they ate the brains of the dead, while the men dined on the rest of the body. The disease itself does not make people more likely to hunger for brains, but the story does drive home a disturbing point: Not only will people eat one another if their wagon train gets stranded in a mountain pass, they will also consume their kin due to cultural forces alone. In other words, peer pressure. Combine a global food shortage with some charismatic and insane leaders, you’ve got yourself a zombie apocalypse.