Considering all the other things we've been told to be afraid of in the past few months (taco trucks on every corner, for instance), we could be at risk of forgetting one of the most popular fictional threats to humanity: zombies. Yes, all right, they're not exactly real, but that hasn't stopped science from getting interested in them. They're not just being macabre; there's more to the science behind zombie apocalypses than you might initially think.
For one, the idea of "spread" of diseases, zombie viruses or not, is pretty scientifically important, because epidemics can kill thousands with or without flesh-eating. For another, zombie-like traits already exist in nature, and have offered a lot of interesting, faintly skin-creeping knowledge to science, raising the possibility that zombification is, while intensely unlikely, not impossible. If zombies ever do emerge, we may be able to have effective models for dealing with them, thanks to research efforts.
Scientists have looked at every aspect of the zombie problem, because priorities. How fast would a zombie apocalypse spread through a city? How could you concoct a zombie virus from scratch? Is it better to run away from a zombie or stay and hit them on the head with a spade? (I'm sure we don't need science to tell us that the correct response is to run like hell.) The result is a quite considerable amount of actual zombie-focused study, which you can add into your zombie apocalypse plan (yes, I know you have one). A lot of it is designed to demonstrate other principles about illnesses, mathematics, or biology, but who cares about that when we have an impending apocalypse on our hands? Knowledge is power, and power is knowing that a zombie virus can take over three million people in an urban area in two months.
Mathematicians Have Modeled What The Spread Of Zombification Would Look Like
Different fictional versions of zombie lore dictate its spread via different means. One is that it's spread by transmission of bodily fluids (i.e. by biting); another is that it's airborne like the flu, though that's rarer. Surprisingly enough, mathematical models of both kinds of zombie transmission through the population actually exist, because zombie infection can stand in pretty effectively for other diseases.
One model, using the three-million-strong population of Chicago for its basis, was just released, and doesn't present a great future: if zombification spreads like a virus, and we don't have an antidote, the scientists behind it predict that it would spread through Chicago like wildfire, taking over the majority of the city's population in just 60 days. That's based on where and how Chicagoans move; the model has also been used to predict the spread of things like flu or ebola through urban areas, based on common hubs of activity and where people tend to reside and congregate. But news isn't necessarily better if it's spread via bite: mathematicians at Oxford modeled that back in 2014, and found that running is a better way to escape infectious zombies than fighting, and that the possibility of friends and family being susceptible means that isolating yourself is a good idea. Hardly a fun outcome.
Zombie-Like Parasites Already Exist In Nature
There are several examples in nature of parasites that take over the bodies of hosts and significantly alter their behavior and habits to benefit the nasty little creepy-crawly now resident in their brain. It's not a mindless cannibalistic horde, but it's as close as current science comes, and it's not very pretty.
They're most common in the insect kingdom: miniature parasites or fungi will target susceptible species, take over their brains and actions, and force them to do things directly counter-productive to their own survival. The fungi Ophiocordyceps, for example, picks certain ants, infects them and then controls them using the release of chemicals, prompting them to fasten themselves on a leaf and die. And the bacteria toxoplasmosis infects rats and mice so that they lose their fear of cats, allowing the bacteria to pass into the cat's digestive system. As of right now, it's not clear that any of these nasties can pass into the human system and cause any mind-control damage.
We Know What A Zombie-Like Illness Might Be Like
Somewhat terrifyingly, part of the international effort against biological warfare includes a knowledge of the possibility that somebody, somewhere, might try to zombify people. And, as Popular Science explained back in 2011, they might have a few ways to do it, though it'd likely be tricky. One option would be prions, the agents behind neurodegenerative diseases like mad cow disease, which could give people significant brain damage while still allowing them to move around; but it'd have to be artificially engineered to give the desired effect. It's also been suggested that rabies could approximate some of the symptoms of zombie-dom; but science has actually got a more specific model up its sleeve, if anybody should choose to work with it.
Carnegie Mellon scientists looked at zombie lore to try and find the main characteristics of a zombie brain, and identify what exactly would have to go wrong to produce a stereotypical zombie "type": shuffling gait, hunger for brains, lack of communication, failure to recognize family and friends, and constant anger. The results were actually pretty complicated; while various aspects of these symptoms can be found in many real-world disorders (the gait, for instance, could be attributed to spinocerebellar ataxia, while damage to the cerebellum could cause the complete lack of memories), the big picture was a complex and unlikely one. The scientists performed the analysis, which became the book Do Zombies Dream of Undead Sheep? , as a model to provide an introduction to neurology, and get into quite involved questions about whether zombies could be described as conscious (probably not) and what happens in the Haitian folklore of "zombification" (paralysis and coercion via drugs, not disease).
So if you're a budding evil genius who wants to create a world of zombies (and why not, as nothing could possibly surprise us any more in 2016), it seems that neurology and the brain are the best places to start. You'll likely end up on some interesting federal watch-lists if you start collecting parasitic fungi, though.