In the zombie flicks 28 Days Later and I Am Legend, an unstoppable viral plague sweeps across humanity, transforming people into mindless monsters with cannibalistic tendencies.

Though dead humans can't come back to life, certain viruses can induce such aggressive, zombie-like behavior, scientists say in the new National Geographic Channel documentary The Truth Behind Zombies, premiering Saturday at 10 p.m. ET/PT. (National Geographic News is part of the National Geographic Society, which part-owns the National Geographic Channel.)

For instance, rabies—a viral disease that infects the central nervous system—can drive people to be violently mad, according to Samita Andreansky, a virologist at the University of Miami's Miller School of Medicine in Florida who also appears in the documentary.

Combine rabies with the ability of a flu virus to spread quickly through the air, and you might have the makings of a zombie apocalypse.

Rabies Virus Mutation Possible?

Unlike movie zombies, which become reanimated almost immediately after infection, the first signs a human has rabies—such as anxiety, confusion, hallucinations, and paralysis—don't typically appear for ten days to a year, as the virus incubates inside the body.

Once rabies sets in, though, it's fatal within a week if left untreated.

If the genetic code of the rabies virus experienced enough changes, or mutations, its incubation time could be reduced dramatically, scientists say.

Many viruses have naturally high mutation rates and constantly change as a means of evading or bypassing the defenses of their hosts.

There are various ways viral mutations can occur, for example through copying mistakes during gene replication or damage from ultraviolet light.

"If a rabies virus can mutate fast enough, it could cause infection within an hour or a few hours. That's entirely plausible," Andreansky said.

Airborne Rabies Would Create "Rage Virus"

But for the rabies virus to trigger a zombie pandemic like in the movies, it would also have to be much more contagious.

Humans typically catch rabies after being bitten by an infected animal, usually a dog—and the infection usually stops there.

Thanks to pet vaccinations, people rarely contract rabies in the United States today, and even fewer people die from the disease. For example, in 2008 only two cases of human rabies infection were reported to the U.S. Centers for Disease Control and Prevention.

A faster mode of transmission would be through the air, which is how the influenza virus spreads.

"All rabies has to do is go airborne, and you have the rage virus" like in 28 Days Later, Max Mogk, head of the Zombie Research Society, says in the documentary. The international nonprofit is devoted to "raising the level of zombie scholarship in the Arts and Sciences," according to their website.

To be transmitted by air, rabies would have to "borrow" traits from another virus, such as influenza.

Different forms, or strains, of the same virus can swap pieces of genetic code through processes called reassortment or recombination, said Elankumaran Subbiah, a virologist at Virginia Tech who was not involved in the documentary.

But unrelated viruses simply do not hybridize in nature, Subbiah told National Geographic News.

Likewise, it's scientifically unheard of for two radically different viruses such as rabies and influenza to borrow traits, he said.

"They're too different. They cannot share genetic information. Viruses assemble only parts that belong to them, and they don't mix and match from different families."

Engineered Zombie Virus Possible?

It's theoretically possible—though extremely difficult—to create a hybrid rabies-influenza virus using modern genetic-engineering techniques, the University of Miami's Andreansky said.

"Sure, I could imagine a scenario where you mix rabies with a flu virus to get airborne transmission, a measles virus to get personality changes, the encephalitis virus to cook your brain with fever"—and thus increase aggression even further—"and throw in the ebola virus to cause you to bleed from your guts. Combine all these things, and you'll [get] something like a zombie virus," she said.

"But [nature] doesn't allow all of these things to happen at the same time. ... You'd most likely get a dead virus."

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