APS News

April 2023 (Volume 32, Number 4)

Controlling a Zombie Outbreak — and Beyond

A modified epidemiology model highlights the role of medical treatment in countering the spread of infections.

By Kendra Redmond | March 16, 2023

blurred crowd people

How many doctors does it take to control a zombie outbreak?

Researchers from Los Alamos National Lab (LANL) and Babeᶊ-Bolyai University in Romania have developed a model for investigating this surprisingly relevant question, which they published in Physical Review E on Feb. 9, 2023.

In most fictional stories of a zombie apocalypse, the reanimated corpses feed on humans. After a bite, humans become ravaging zombies looking for their next meal. With no medical or magical intervention, all people are eventually zombified.

Less the drama of plot twists and characters fighting for survival, the zombie apocalypse tells the story of an uncontrolled epidemic. And, like other diseases, its spread can be mathematically modeled. In terms of modeling, an epidemic is “a chain reaction on a set of interacting particles,” explains Nicholas Hengartner, a senior scientist at LANL and coauthor of the new paper.

Researchers have applied epidemiology models to a zombie outbreak for more than a decade, and it doesn’t look good. In this latest work, led by Cynthia Olson Reichhardt, also a senior scientist at LANL, researchers modified the susceptible-zombie (SZ) model — or, more generally, the susceptible-infected (SI) model of an epidemic. This model applies to lifelong infectious diseases for which there is no recovery. In an SI epidemic, the number of uninfected people always falls to zero over time. Victory for the zombies.

Typical SI models treat people like molecules in an ideal gas, bumping into each other randomly, explains Hengartner. “But there is some clustering in how we interact,” he says. To capture this, the team treated people (and zombies) as active matter — as self-propelled, interacting particles that behave like birds in a flock or fish in a school — where clustering naturally emerges. The team also added a new category of susceptibles to the mix: Clerics (C). Clerics could heal infected people or reduce the rate of infection, but they could also be infected. The researchers call their model the susceptible-cleric-zombie-recovered (SCZR) model.

They ran the SCZR model for different infection rates, recovery rates, and numbers of clerics. Its predictions revealed two fundamentally different paths. When the number of clerics or recovery rate was low, the SCZR epidemic behaved like an SI epidemic and the zombies prevailed. But when the number of clerics or recovery rate was high, the clerics healed all the zombies to save humanity. The SCZR model allows you to study how health care resources affect the outcome of such an outbreak, Hengartner explains.

Clerics are a fascinating addition to the SI model, says Stacey Smith?, a math and statistics professor at the University of Ottawa and coauthor of the first zombie apocalypse modeling paper, published in 2009 (the question mark is part of her name). Adding some clerics to deal with zombies one-on-one might seem like a small intervention, she says, but you can “pull the levers of how effective your clerics are” to swing from one outcome to another.

Whether clerics could actually heal zombies is up for debate; most universes don’t allow the undead to come back to life. But that doesn’t really matter to the researchers. The zombie scenario is an interesting way to encourage out-of-the-box thinking and raise awareness, Hengartner says. The team’s underlying goal is to explore the impact of medical intervention on the infection rate of diseases like HIV and hepatitis C. In these cases, people who are untreated can infect others over a lifetime, but enough health care providers and resources can move the infection rate toward zero.

Physical Review E journal image
Credit: Derived from A. Libál et al. / Phys. Rev. E 107, 024604 (2023)

The susceptible-cleric-zombie-recovered model over time. Susceptibles are yellow, clerics are brown, zombies are green, and recovered are blue. (b) and (e) show the peak of the zombie outbreak. (c) and (f) show the final state. In (a), (b), and (c), all susceptibles and clerics are eliminated in the final state; in (d), (e), and (f), all zombies are eliminated in the final state.

The researchers are currently expanding the SCZR model and working with collaborators to apply it to HIV data. If it’s a good fit, Hengartner is optimistic that the model could help researchers, public health officials, policymakers, and others find answers to essential questions: How many doctors do we need to control an outbreak? How many treatment kits? And what kind of investment could change the outcome of this epidemic?

Kendra Redmond is a writer based in Minnesota.

APS encourages the redistribution of the materials included in this newspaper provided that attribution to the source is noted and the materials are not truncated or changed.

Editor: Taryn MacKinney

April 2023 (Volume 32, Number 4)

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Articles in this Issue
This Summer, Particle Physicists Will Prioritize Projects for the Field’s Future
Controlling a Zombie Outbreak — and Beyond
Get Ready for the APS April Meeting 2023
This Month in Physics History
To Manage Future Risks of Emerging Technologies, Train Physicists in Ethical Decision-Making Now
What the James Webb Space Telescope Can Teach Scientists About Engaging the Public
Opinion: Biological Physics Comes of Age
APS Members Advocate for Science on Capitol Hill
Congress Picks New Leaders for Key Science Committees
Letter to the Editor