Design your own apocalypse (Image: ZihniIoglu Kamil/SIPA/REX)

It began in Chicago. One by one, other cities fell. As the dead began to sweep across the nation, only those in the most remote locations remained safe. Soon, just a few desperate souls would remain alive.

That’s what happened when I started my very own zombie outbreak, thanks to a model examining how an undead plague might ravage the United States. You can try it for yourself here.

Alexi Alemi of Cornell University in Ithaca, New York, and his colleagues began developing the model, which uses tools from disease modelling, as a class project after reading the novel World War Z. They aren’t the first researchers to turn their hand to zombie maths – there’s even a whole book on the subject – but Alemi wanted to up the challenge by using US census population data. “It felt like a natural challenge to simulate a ‘realistic’ zombie outbreak,” he says.


The team took a classic epidemic model called SIR, in which people are either susceptible to, infected by or recovering from a disease – then they added a zombie twist. “For normal diseases you either get better on your own, or you might die,” says Alemi. “Zombies don’t get better, nor do they die, so the only way you can get rid of a zombie is for a human to actively kill it.”

28 Days Later

This change meant the outcome of the model depended on a single parameter: the ratio between bites and kills. This is a measure of how effective humans are at fighting off zombie attacks. From studying films like Shaun of the Dead, they decided the most realistic bite-to-kill value was 0.8 – in other words, zombies are around 25 per cent more likely to bite humans than humans are to kill zombies – but in the online simulator you can tweak that value.

With the basic model in hand, the team turned to how the infection might spread geographically. They created a grid over a map of the US and populated each cell in the grid with a certain number of humans, taken from the population of the 2010 census.

Running the simulation, they found that it didn’t seem to matter where the outbreak started. From most locations it took around a month – or 28 Days Later – for most of the US to succumb to the plague, but some remote areas of Montana and Nevada remained zombie free even four months later . Alemi will present the results at the American Physical Society meeting in San Antonio, Texas, on Thursday.

Fortunately, zombies are fictional, but the research could also help people modelling real diseases. It turns out the equations in the zombie version of SIR are actually easier to solve than the real thing, so lessons learned from creating the model may translate back to real-world research, says Alemi.