Were there ever to actually be a Walking Dead-style zombie apocalypse, most of the planet’s human population would be wiped out in a mere 100 days, according to a new study published earlier this week in the University of Leicester’s Journal of Physics Special Topics.

In fact, thanks to their creative use of the SIR model (a basic epidemiological model which is typically used to simulate how contagious diseases spread), a team of undergrad students at the university determined that each zombie had a 90% chance of finding and infecting at least one new victim each day, and that the undead could go 20 days without feeding on brains.

Furthermore, according to CNET and LiveScience, the authors concluded a zombie would be able to survive 20 days, and that if just one of them was able to catch an estimated population of 7.5 billion humans people unaware, it would take less than three weeks for the walking dead to become a noticeable epidemic. After 100 days, there would be only a handful of humans left to deal with a zombie population of more than 190 million, the study determined.

However, the authors wrote that natural birth and death rates were omitted, as the short term of the anticipated zombie rendered them “negligible compared to the impact of the zombie virus over the short time frame.” They also noted that they “have also not included the possibility for the humans to kill the zombies. Including this may give the humans a better chance at survival.”

Odds of survival increase when we fight back and reproduce

Those might seem like pretty significant oversights, and as LiveScience noted, a more realistic model would likely assume that the odds that a zombie would be able to find “fresh meat” would likely decline as the number of surviving men and women began to plummet over time.

Fortunately, the Leicester researchers published a second paper addressing some of those issues. They started by increasing the zombie life span to one year, but gave each human a 10% chance of exterminating one of the undead every year and factored in human reproduction at a rate of a new infant being born to each reproductive-age female once every three years.

This altered scenario “made human survival more feasible,” the study authors said in a statement. Once again, the human population experienced a significant drop off during the first 100 days of the zombie apocalypse. However, after 1,000 days, the zombies became extinct, and an estimated 10,000 days after the start of the apocalypse, the human population would start to recover.

“Every year we ask students to write short papers for the Journal of Physics Special Topics, Dr. Mervyn Roy, a lecturer in the university’s Department of Physics and Astronomy, explained in a statement. “It lets the students show off their creative side and apply some of physics they know to the weird, the wonderful, or the everyday.”

As LiveScience noted, this is hardly the first scientific exercise to focus on a potential zombie apocalypse. In December 2015, the British Medical Journal, used the spread of the undead as a way to raise awareness for how real-life pandemics spread, and the Centers for Disease Control and Prevention (CDC) employed a similar tactic in a blog post of its own.

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