"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.
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.
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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.