Zombie Apocalypse Possible.

SOURCE: National Geographic.

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.

(Related: "New, Fast-Evolving Rabies Virus Found—and Spreading.")

"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.