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How Zombies Can Save Us from a Real Apocalypse

How Zombies Can Save Us from a Real Apocalypse

By Nick Beeton, Alexander Hoare & Brody Walker

Mathematical modelling of a zombie apocalypse has real-world applications in our responses to infectious diseases such as Ebola and HIV, wildlife conservation and even the teaching of statistics.

Zombies are big right now. They’re on your TV screens (The Walking Dead), in your computer games (Plants vs. Zombies) and have even snuck into classic literature via such books as Pride and Prejudice and Zombies. They’ve become one of those internet memes that just won’t die; people seem ready to squeeze them into every conceivable situation.

It’s probably not surprising that zombies have also attracted the attention of scientists. After all, we’re traditionally a pretty geeky bunch. As geeky scientists ourselves, we contributed a chapter to a book called Mathematical Modelling of Zombies. Its chapters cover a range of potential issues around a hypothetical zombie apocalypse.

Although some might consider this kind of work a waste of time, what is surprising is that studying zombies can actually have real-world benefits. Among other things, preparing for a fictional zombie apocalypse can help us plan for diseases that, despite being very real, are more similar to being besieged by a plague of the flesh-eating undead than you’d think.

In real life, deadly infectious diseases don’t usually cause the extinction of their host species. To be able to sustain itself, a disease needs to be able to infect, on average, at least one new individual every time it finds a host. To do this it needs to be infectious enough to be able to spread, but not too immediately deadly or it will kill or injure its hosts too quickly, leaving it unable to spread further.

What usually happens in a human population is that eventually people will work out how to stop the infection’s spread, either by finding a cure or by isolating people who are potentially infected. For the current Ebola epidemic there is no cure available yet, and reliably isolating and treating infected people is difficult as it is transmitted via blood and bodily fluids.

Although wildlife populations can’t do either of these things without human intervention, such diseases don’t usually drive them to extinction either. This is because of what is called the “threshold host density” – a point at which the disease has ravaged a population so badly that there aren’t enough individuals left to continue spreading it. At this stage the disease dies out and the population is able to recover.

Unfortunately, things aren’t always so simple. For example, the Tasmanian devil is currently threatened by devil facial tumour disease (DFTD), an infectious cancer that is transmitted from devil to devil through bite wounds. This disease, unlike most, continues to spread even at very low host densities because of their biting behaviour: not only do they bite in their usual social and feeding interactions but they also bite each other while competing for mates – and even during sex. As any recipient of the awkward parental talk knows, sex is necessary for the species to continue surviving, so as long as devils are breeding the infection will continue to spread. This makes the disease far more likely to cause extinction in the long term, and far more difficult to treat. The devils are an example of a disease where the natural behaviour of the host can make a bad situation worse.

There are also cases where the infection itself can cause the host’s behaviour to change. Perhaps the most famous of these is the parasitic disease toxoplasmosis. As part of its life cycle, the parasite Toxoplasma gondii alters the behaviour of mice and rats in such a way that they are no longer averse to cat odours. Their host being eaten by a cat is vital for the parasite, as it is only able to sexually reproduce inside cats. There is evidence that the parasite even changes the behaviours of humans who become infected, making them more likely to do risky things.

These examples are part of why behaviour is a big part of the puzzle of disease. A zombie infection takes this idea to its extreme: it instantly changes an ordinary person into a mindless machine, motivated only to spread the infection to others.

Our research showed that a zombie plague does not behave like any of the traditional models for human or wildlife disease. This tells us that in order to be able to predict the spread of any disease, we need to understand the behaviour of the people who may fall victim to it.

This kind of research is par for the course with human disease modelling. For example, HIV/AIDS research involves extensive surveys of sexual behaviour and drug use. It is becoming more common in wildlife disease modelling as well – technology to track and monitor individual animals is continually improving. Recent research has even been able to track the movement of individual bees in hives to help study their recent declines.

We can use this information to help design strategies to limit the spread of disease. For HIV, this might take the form of awareness campaigns about safe sex and safe drug use. For Ebola, both awareness campaigns and providing resources for the isolation and care of victims are part of the solution.

For zombies, the traditional solution is of course removing the zombies by any means possible. We researched the effects of zombie eradication in our chapter, and found that around 10 armed people per 100 unarmed people was enough to stop an outbreak.

Even this nightmare scenario has parallels in real life. Some wildlife diseases are controlled by removing or culling infected individuals to reduce the rate of transmission. In most cases this is unsuccessful, such as Tasmanian devils with DFTD or badgers with tuberculosis in the United Kingdom. Our results suggest that the culling approach is far more useful with zombies than with wild animals, partly because the zombies are much easier to find because of their attempts to track us – an unexpected advantage of us being their prey and predator at the same time.

With all these applications, it’s not surprising that zombie modelling is also of interest to teachers. Statistics and data handling are an essential part of Australia’s national curriculum, but are often taught using examples that the students can’t relate to. On the other hand, most high school students have at least some knowledge about zombies.

Mixing zombies and statistics is the natural conclusion. No longer do you have to force students to pay attention as they eagerly attempt to make the best zombie hunters or work out how many zombies can “eliminate” people the most efficiently. This is far more interesting stuff than the usual textbook examples.

To this end, Mathematical Modelling of Zombies contains a variety of mathematical modelling approaches that could be suitable for teaching students at a range of different levels. Who would have thought that zombies could help save the world from diseases and teach maths at the same time?

Nick Beeton is a Postdoctoral Research Fellow at The University of Tasmania’s School of Biological Sciences. He is co-author of a chapter in Mathematical Modelling of Zombies with Alexander Hoare and Brody Walker.