What is a zombie?

If I’m going to analyze zombie behavior, the first step is to lay down some ground rules. Or axioms, whatever. So the first question is, what is a zombie?

Well, there are a lot of conflicting definitions for that. The original zombie modeling paper actually has a decent overview of how the definition of a zombie has varied through the ages. According to that, the modern "standard" comes from Night of the Living Dead, which portrays zombies as mindless, soulless creatures who move slowly and have a singular fixation on consuming human flesh. For analytical work, this definition of a zombie is kind of nice because it’s easy to model. You can easily program it into a computer; the logic to identify the closest human flesh and walk toward it isn’t very complicated.

On the other hand, there are plenty of counterexamples where the zombies are quick and even intelligent, like Zombieland (which I am a big fan of, by the way). Unsurprisingly, there are models for this sort of thing too. Environmental biologists use them to study predator-prey relations, and fictional mathematicians use them to catch escaping criminals ;-) (Actually, to be fair, so do real mathematicians sometimes — a lot of those plot lines are based on true stories) These sorts of models tend to be more complicated, as you’d expect.

Anyway, for now I think it makes sense to start simple. With that in mind, this will be the First Operational Definition of a zombie (capitalization is totally tongue-in-cheek, but at some point I might expand this into a real zombie classification system). It’s basically a list of characteristics of the simplest possible model that corresponds to something people would think of as a "zombie."

  • Zombies are slow. Specifically, a zombie’s top speed is slower than a person’s top speed. Zombies always move at their top speed, when they are moving at all.
  • Zombies are dumb. A zombie moves in a random path, until it gets within some distance of human flesh, and then it makes a beeline for the flesh.
  • Zombies are formed from (relatively) intact human corpses. This means that any process which produces a zombie must begin with the loss of a living person. The requirement of an intact human corpse implies that it is possible to kill people in such a way that they cannot be zombified.
  • Zombification agent is transmitted by human-zombie contact. I’m going to assume, as is typical in the movies, that there is some infectious agent (like a virus) that causes a human to become a zombie after his/her death. This infectious agent can only be transmitted by some particular kind of contact between a living human and a zombie (e.g. a bite). Of course there has to be an exception, some initial "seed" process that produces at least the first zombie, but I’ll assume that that is relatively localized and short-lived.
  • Zombies can be killed. This one seems to be rather contentious, at least in certain circles, but I think most of the primary sources (movies and books) are in agreement: there is some way to kill a zombie such that it cannot possibly be reanimated. Sort of ties into the intactness requirement from the corpse point.

There’s a problem with this list, though: if zombies are slow and dumb, wouldn’t everyone see them coming and run away? How would you ever get the direct physical contact needed to transmit a zombie infection? Well, you probably wouldn’t, in reality. But since this is a super-simple zombie model, I’m going to make an equally simplifying assumption about people: we’re also dumb. Not that people move toward the closest living flesh, but we do kind of move in random paths. I suspect that if you compared a model of a bunch of people moving randomly with actual data on the movements of people in a large city, you’d find that on a large enough scale, they’re fairly similar, at least for the purposes of modeling infectious disease transmission. And I also suspect that some people in a real major city could be kind of oblivious to a zombie roaming the streets, until it was too late for them. (Not to mention, you know there’d be zombie biologists who would insist on studying live samples)

But anyway, it’s just a model, and just a starting point. What I hope to do in the future is refine the assumptions of this model to make more realistic ones, and thereby get better and better approximations to a real zombie apocalypse.

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