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How would a zombie invasion play out in the US? These scientists modeled it.

If a Walking Dead-style zombie outbreak ever swept across the United States, your best bet would be to head for the northern Rockies.

That's one upshot of some fun 2015 research by Alex Alemi, Matthew Bierbaum, Christopher Myers, and James Sethna of Cornell University. After reading World War Z, the group wanted to figure out how a real-life zombie apocalypse might play out in the United States.

So they did the natural thing and modeled it. They've even created a zombie simulator that lets you see for yourself how the invasion would spread, based on different assumptions. Try it!

I placed a lone hungry zombie in New York City, and, within a week, the invasion had spread throughout the East Coast, overrunning Boston and Washington, DC. And those red splotches kept growing… and growing…

28 days later, much of the eastern US was conquered — though, surprisingly, not all of it. The undead hadn't yet made it to Chicago or Florida. Maybe if they had been faster, it would've all been different...

The zombie simulator is based on a real-life disease model known as SIR (which stands for "susceptible, infected, and resistant"). The researchers developed simulations based on variables like how fast zombies move, how often humans kill them, and how often bites occur. The team is presented its results at the 2015 American Physical Society March Meeting.

"Modeling zombies takes you through a lot of the techniques used to model real diseases, albeit in a fun context," Alemi said in a statement. "Each possible interaction — zombie bites human, human kills zombie, zombie moves — is treated like a radioactive decay, with a half-life that depends on some parameters, and we tried to simulate the times it would take for all of these different interactions to fire, where complications arise because when one thing happens it can affect the rates at which all of the other things happen."

A lot depends on zombie speed and the "bite-to-kill" ratio

Chomp, chomp. (Sean Gallup/Getty Images)

The simulator allows you to control various factors, like the "bite-to-kill" ratio — a measure of how likely zombies are to bite humans versus how likely humans are to kill zombies. (The researchers told Jacob Aron of New Scientist that an 0.8 bite-to-kill ratio was roughly the value they found for movies like Shaun of the Dead, though you can tweak accordingly if you're more bullish on the human race.)

When I ran the zombie simulator using the default assumptions, it took several months for zombies to overrun the entire United States. Cities fell astonishingly quickly. But zombies were much slower to spread into rural areas. And areas like the northern Rockies remained zombie-free for a long time. The researchers say this usually holds true no matter where the initial zombie outbreak begins.

"Given the dynamics of the disease, once the zombies invade more sparsely populated areas, the whole outbreak slows down — there are fewer humans to bite, so you start creating zombies at a slower rate," Alemi said. "I'd love to see a fictional account where most of New York City falls in a day, but upstate New York has a month or so to prepare."

How long could those rural pockets hold out? It depends whether hungry zombies would be able to hang around indefinitely. On that score, I've always found this piece by David Dietle at Cracked fairly convincing — given enough time, zombies would succumb to heat, cold, natural predators, maggots, and an inability to heal from day-to-day damage. Holing up for a month or so in the Rockies should be enough to let the zombie apocalypse play itself out.

Or at least that's the hope.

Further reading: Alemi has a more detailed explanation of his research here. he also discusses ways to improve the model: "Given the time, we could attempt to add more complicated social dynamics to the simulation, such as allowing people to make a run for it, include plane flights, or have an awareness of the zombie outbreak, etc."

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