In the new movie World War Z, a zombie virus has overtaken the planet, killing off most of humankind and leaving (who else?) only Brad Pitt to save the rest of us.

Inspired by the 2006 novel of the same name, the film’s only the latest example of America’s zombification. In researching this article, I learned that at least one hospital—California’s Sutter Roseville Medical Center—has had a zombie apocalypse drill, that the U.S. Centers for Disease Control and Prevention (CDC) has a preparation guide for the living dead, and that conservationists are thinking about how nature can help us if the world were to fall to zombies. Walking to the movie theater in downtown Washington, D.C., I was even randomly handed a flier for “How to Survive a Zombie Apocalypse” by a religious group.

And, after three seasons of Walking Dead, I considered myself fairly zombie proficient. But I was still awestruck by World War Z.

Unlike in Walking Dead, these zombies are gasp-inducingly fast, come to life seconds after being bitten, and make high-pitched, almost bird-like noises. The suspenseful moments—especially when the characters tiptoe through a zombie-filled laboratory—will keep even the most jaded moviegoer engaged, and the aerial scenes of tens of thousands of zombies swarming over iconic cities are purely fascinating to watch.

Also surprising: Unlike the novel, in which UN investigator Gerry Lane (Pitt) travels the tatters of civilization interviewing survivors, the movie is set at the beginning of the apocalypse, and Lane goes from place to place to try to stop the zombie virus.

The solution, he discovers, is to “camouflage” people by making them sick with another microbe, under the premise that zombies have no interest in attacking you unless you’re healthy. Skeptical, I asked Joan Slonczewski, a microbiologist at Kenyon College in Ohio, if that’s how viruses really work.

No, at least in a person or animal’s body: “I’ve never heard of a case where virus infection prevents transmission from another person or animal,” she said.

But, at the cellular level, “it is possible for one virus infecting a cell to prevent ‘superinfection’ of that cell by another virus. For example, herpes infection of a cell may prevent superinfection of that cell by another herpes strain.”

Zombie Virus Possible?

As a science fiction writer, Slonczewski is used to imagining doomsday scenarios. Her 2010 novel Brain Plague featured zombie-like characters that bite people to transmit intelligent microbes, which then communicate with the infected person’s brain cells. So I asked her: Could a zombie-like virus occur in real life?

Sort of. No one’s expecting the dead to rise again, of course, but there are so-called neurotropic viruses that attack our brains and cause aggressive or bizarre behavior.

“I suppose you could imagine a [new neurotropic] virus that would cut off the higher brain function and then induce a starvation-like state and could thereby induce the disabled person to go after brains”—the classic zombie meal, Slonczewski said.

Some existing neurotropic viruses are already the stuff of nightmares. For instance, “rabies is pretty scary on its own,” said Kartik Chandran, a microbiologist and immunologist at the Albert Einstein College in New York who studies the deadly Ebola virus. (In the movie and book, the zombie plague is first misidentified as rabies.)

Like the mythical zombie virus, rabies is transferred through biting. Once inside your body, the virus travels directly to your brain and “makes you go nuts and go and bite more people,” he said. More than 55,000 people, mostly in Africa and Asia, die from rabies every year, which is one person every ten minutes, according to the World Rabies Day website. Most infected people are bitten by rabid wildlife. (See “New, Fast-Evolving Rabies Virus Found—And Spreading.”)

Making of a Monster

How would a “zombie virus” arise? One possibility is that two viruses could join together and form a hybrid.

Viruses work by copying their genetic material within human cells. If there are two viruses in the same cell, one virus may accidentally jump on a genetic copy that belongs to the other virus. The progenitor to the human HIV virus may have arisen this way in Africa when a chimpanzee and monkey virus combined, Chandran explained.

I also asked Chandran if the Ebola virus—which causes widespread hemorrhaging—and rabies could ever hybridize and infect people, making them look and act like zombies. He said that’s unlikely because the viruses aren’t closely related, and that even if it were to happen, the resulting “bastard progeny would be nonfunctional or poorly functional.”

But there’s another way that a mutant virus can arise: if there’s a glitch in the genetic copying machinery of an existing virus. Sometimes, one of these mutations confers an advantage to the virus that allows the mutated strain to outcompete others and quickly take over the world. (Take an infectious diseases quiz.)

Called a “selective sweep,” this usually happens frequently with influenza. (See video: “How Flu Viruses Attack.”)

That’s why the concept of a World War Z-like viral plague is not so far-fetched.

Viruses also succeed when they encounter populations that have no immunity against them. A good example is measles in the New World in the 1600s, noted Chandran. Europeans, who were resistant to measles virus, brought it with them to the Americas, where the natives weren’t. It spread like wildfire.

Viruses Not All Bad

But before you panic about a zombie apocalypse, remember that not all viruses are bad. Slonczewski noted that there are more helpful viruses than harmful ones. Some viruses are even essential to our survival, performing functions like stimulating our immune system.

Chandran agreed that “viruses are indispensable to life as we know it.” For instance, viruses shuffle nature’s genetic deck by constantly moving new genetic material from species to species.

Another thing to consider: The majority of viruses on Earth infect single-celled microbes, and have no interest in us humans.

“We’re kind of a sideshow. This whole planet’s really about microbes,” Chandran said, “just waiting to take it back.”

That should put your mind at ease.

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