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Could a Zombie Apocalypse Really Happen?

Zombies have a special place in pop culture at the moment. Featured in countless comics, TV shows, and movies, they have a certain allure that is hard to pin down, but is entertaining all the same. Yet, most people who consume such material never really think that any of it could ever be actually real. After all, this is the world of fiction, right? The realm of the imagination and squarely out of the domain of the real. Right? Unfortunately there has been such speculation on how an actual real zombie outbreak could go down, and in many cases it is well grounded and based on plausible scenarios backed up by science as we know it. Let’s take a look into the world of the possible real zombie apocalypse.

One way a zombie outbreak might occur is through a virus or other disease that serves to dramatically alter the behavior of its host. Perhaps the best known example is that of the rabies virus, a highly virulent and deadly disease that infects a wide range of hosts and is spread through bodily fluids such as saliva, typically in the form of bites from infected animals. The rabies virus does not have a complex lifecycle and is fairly indiscriminate in which hosts it infects, instead incurring a fitness advantage simply by spreading itself as far and wide as possible in a short period of time. Upon infection, the host exhibits a wide range of symptoms as the disease acts upon the central nervous system, including anxiety, restlessness, confusion, hallucinations, paralysis, involuntary muscular contractions, uncontrolled perspiration, salivation, and pupil dilation, and hydrophobia (fear of water), which usually make themselves known after an incubation period of around 10 days.

The most dramatic symptom of rabies is a marked increase in agitation, anger, and aggression, a trait that makes infected animals more likely to bite and thus increase transmission of the virus. Within a period of mere days after exhibiting symptoms, the victim will fall into a coma and die if not treated. Although mostly found in animals, it is possible for a human to contract the disease through a bite from an infected animal, and if it is not treated in a timely manner they will invariably show all of the same symptoms, including the heightened anger and aggression, although our acute self-awareness allows us to somewhat control our primal urges to lash out and bite everything that comes near us, and there has never been any confirmed case of human to human transmission of rabies as of yet.

Spookily enough, if the rabies virus were to mutate fast enough and in a way that allowed it to dramatically shorten its incubation period, decrease its fatality rate in order to allow more time to spread, and additionally become airborne, it could very well turn into something that resembles a full-blown zombie pandemic from a horror movie. Although rabies is now transmitted only through bites from infected animals, for it to become an airborne contagion is not a completely far out notion. All it would have to do is swap some of its genetic material with an airborne virus such as influenza, a process known as “recombination,” and something that different strains of the same virus do all of the time, although rabies and influenza are far too dissimilar for this to feasibly occur in nature. Or it could simply be engineered that way by some nefarious party.

With a very short incubation period, low fatality rate, and highly contagious means of spreading, and perhaps even more acute symptoms of aggression, carriers of the virus would essentially become mindless, furious vectors for the pathogen roiling within them, and it would spread beyond our ability to control it. A professor in the Department of Cell Biology and Molecular Genetics at the University of Maryland, Jonathan D. Dinman, PhD, summed it up nicely in an interview with RedOrbit:

So, you start with Rabies virus, but you engineer it so that it doesn’t actually kill you. It just takes over your brain and makes you want to bite other people to spread itself. Infected people just become automatons devoted to spreading the virus. The main viral property you’d want to change would be to convert it from causing an acute infection (like Ebola which tends to kill the victim quickly) to persistent infection (like Herpes, which stays with you for your entire life). Functions you’d want the infected person to retain would be metabolism (so they can produce more virus) and motility (so they can get from victim to victim). You would want the virus to cause infected people to lose the ability to think independently (and therefore come up with a cure).

Rabies is certainly not the only disease that has the potential to produce zombies, and there are other diseases like this that affect behavior as well. One is called encephalitis lethargica, which can cause symptoms such as high fever, sore throat, headaches, hallucinations, atypical behavior and psychosis, tremors, seizures, and eventually a listless, nearly catatonic state in which the victim remains speechless and motionless, as if unaware of the world around them. Even if one is to survive the illness they typically never quite return to normal, and Oliver Sacks said of these patients in his book Awakenings thus:

They would be conscious and aware – yet not fully awake; they would sit motionless and speechless all day in their chairs, totally lacking energy, impetus, initiative, motive, appetite, affect or desire; they registered what went on about them without active attention, and with profound indifference. They neither conveyed nor felt the feeling of life; they were as insubstantial as ghosts, and as passive as zombies.

Making it all even stranger still is that it has been reported that despite this seemingly docile catatonic state, victims suffering from the disease are sometimes known to suddenly and violently lash out at physical stimuli as innocent as being simply lightly tapped on the shoulder. It certainly sounds very much like zombies to me. Such viruses might not cause a literal zombie apocalypse now, but what if they were to be manipulated or engineered to such an effect?

Perhaps it would not even take some shadowy organization to concoct such a disease, but rather an entirely different sinister force; Mother Nature herself. Viruses are constantly mutating in nature, and some of these naturally occurring lethal pathogens, such as Ebola, Anthrax, and many others, are every bit as diabolical and deadly as something cooked up in a lab. Nature has a certain knack for coming up with truly wicked ways of invisibly killing us that are often frustratingly difficult for us to combat or fully understand. A line from the movie World War Z sums it up pretty well:

Mother Nature is a serial killer. No one’s better, or more creative. Like all serial killers, she can’t help but have the urge to get caught or what good would all those brilliant crimes do if no one takes the credit? So she leaves crumbs. Now the hard part, is seeing the crumbs, the clues there. Sometimes it’s in your thoughts where the most brutal part of a virus is. Turns out to be the chink in its armor. And she loves disguising her weaknesses as strengths. She’s a bitch.

Another potential way to trigger something akin to a zombie apocalypse in humans would be to artificially alter our brain in such a way as to induce an explosive predatory response or to diminish our fear and emotions. Somewhat spookily, this has already actually been on mice in a laboratory setting and the results are rather frightening.

The study was carried out by neuroscientist Ivan de Araujo, from Yale University, along with colleagues Wenfei Han, also of Yale, and Milton Canteras, of the University of São Paulo, and its main purpose was to determine what parts of the brain were responsible for certain motor responses in the animal by studying brain functions that control predatory behavior. The main focus here was a part of the brain called the amygdala, which governs fear and emotional responses, as well as apparently holding motor controls for capturing and killing prey, and they went about tinkering with the neurons there, of which Araujo said, “We targeted the groups of neurons that control predatory hunting, pursuit, capture and killing.” Essentially, they wanted to see if they could induce predatory hunting behavior in these lab mice, which would turn out to be more successful than they may have imagined.

The scientists went about using light to manipulate the targeted neurons, in a technique known as “optogenetics,” in this case via a sensory device equipped with light emitting optic fibers attached to the heads of the mice who had been genetically engineered to have neurons that would respond to the blue light. When the light was turned off the mice were docile and calm, but as soon as the light was switched on they underwent a radical change, transforming to ravenous killing machines lashing out to attack and repeatedly bite anything they could find, including crickets, plastic toys, sticks, and even bottle caps. In addition, the light seemed to give them super biting strength, of which Araujo said:

Their bite force is kind of powerful and pretty sufficient. When we stimulated these neurons, we observed that the muscles were contracting much more strongly, the bite force was more powerful.

This response led to researchers finding that there were two responses coming from the amygdala, one for chasing down prey and one that controlled muscles in the jaw and neck for delivering the killing bite, and it illuminated more on the exact capabilities of this particular region of the brain. Interestingly, this violent response seemed to have some limits. For instance, it seemed to be linked to hunger, with hungry mice responding much more viciously than mice that were full, and the mice did not attack each other or anything unreasonably large. It is thought that triggering this part of the brain is opening a sort of gateway that is usually closed to certain behaviors lurking behind it that are normally suppressed or blocked by the amygdala. Peter Cummings, a neuropathologist at Boston University who has looked into the scientific possibility of zombies and written the book The Neuropathology of Zombies, has said of this:

If I spilled coffee on you, you might want to punch me in the face. That’s due to the limbic system. But usually the frontal lobe shuts that response down. But if you lose that connection, the amygdala takes over and that response takes over.

Of course considering that humans have a very similar organ in our brains is it feasible that someone could further perfect this “switch” and use it to manipulate the brain into turning us into mindless lunatics? Stranger things have happened. It is easy to imagine someone finding such a switch useful for military applications, so although this has only been seen in mice so far and is far from completely understood, it is rather frightening to think about what would happen if someone were to start experimenting with this on us.

Damage or conditions of the brain can cause the same sort of effects, as can be seen with a condition called “Klüver–Bucy syndrome,” which is caused by lesions on the medial temporal lobe of the brain. This causes a wide array of bizarre symptoms, many of which could be seen as quite zombie-like. Victims of the syndrome can exhibit amnesia, dementia, loss of concentration, inability to recognize objects, hyper sexuality, loss of fear, loss of coordination, increased docility or conversely aggression, lack of emotional response, and the desire to eat inappropriate or strange objects or to put these objects in their mouth. Seeing someone with this rare condition could very well make you think they were a zombie.

Besides viruses there are other ways that a zombie outbreak might occur, such as parasites that effect the brain in such a way as to profoundly affect the host’s behavior or even turn them into a real zombie. This is actually already present in nature among some organisms, with the most well-known example being that of a parasitic fungus from the genus Ophiocordyceps. This fungus invades the brain of an ant, and reprograms it to be overcome with the obsessive desire to climb to the highest point it can, after which it will be compelled to clamp down on a leaf or branch as the fungus grows within it, ultimately exploding from the ant’s body to send spores everywhere and start the cycle anew.

A rather gruesome and infamous example is a type of hairworm that infects grasshoppers and takes over their nervous system to compel them to commit suicide by jumping into pools of water, after which the hairworm rather explosively and horrifically spews forth from the host like something straight out of a horror movie to continue its life cycle in the water. There are actually quite a lot more behavior altering parasites like this in nature, overriding the host’s normal brain activity in order to make it do what it wants, and although there are none that are known to affect human beings it is not a stretch to think that one may mutate do so, either naturally or through genetic engineering.

Another classic example of this is the lancet liver fluke, Dicrocoelium dendriticum, which is typically found in the livers of grazing mammals such as sheep or cattle. The first intermediate host is land roving mollusks, such as slugs or snails, the slime trails of which leave behind the parasite, which are then ingested by ants. The next step is for the parasite is to make it into an animal such as a cow, but how to get there? In short, zombification. The parasitic larvae make their way to the ant’s brain, where they encyst and create a strong urge for the ant to climb to the highest blade of grass it can find at night. Upon reaching the top, the cysts cause an involuntary spasm of the mandibles of these “zombie ants” through the release of tetanus, which causes the ant to bite down hard on the grass and prevents it from going anywhere. In the morning, a grazing mammal comes along, eats the grass with the immobile ant on it, and the lifecycle is completed.

Other examples of such host manipulation can be found with thorny-headed worms, which alter the neurotransmitters of their hosts’ brains to achieve the desired result of moving on to birds from its initial host, a type of crustacean known as a gammarid. These gammarids typically live in ponds, and their natural reaction to surface movement or disturbances is to seek darkness and to dive into hiding beneath mud on the bottom. However, when infected with thorny-headed worms, they instead swim up towards light when there is a disturbance at the surface, where they are eaten by birds and therefore complete the parasite’s life-cycle. The worms achieve this by producing a strong immune system response that releases large amounts of the neurotransmitter serotonin, which then disrupts signals from the eyes to the brain and likely tricks the gammarids into confusing up for down and light for dark.

There are several types of parasitoid wasps that also manipulate their hosts in shockingly sophisticated ways. Parasitoid wasps typically lay their eggs in a host, after which the larvae will eventually hatch and feed upon it. One type of wasp native to the rainforests of Costa Rica targets the species of spider Anelosimus octavius as its host. This type of spider generally weaves knotted, gnarled webs of haphazard threads, but infected individuals will go off and spin webs that are completely different in design. Infected spiders essentially have their brains hijacked into weaving webs that have a platform covered by sheets of webbing that protect it from the rain and elements, as well as an odd opening at the end of the platform. When these bizarre webs are finished, the wasp larvae emerges from the spider, killing it, makes its way to the end of the well protected platform, and hangs from a cocoon through the opening that the spider has graciously provided. It is extraordinarily creepy that this parasite can not only alter its host’s behavior, but also essentially have it act as its own personal architect.

A parasitoid wasp

The parasitoid wasp, Glyptapanteles, also demonstrates a particularly dastardly form of mind control on its host, actually turning it into a guardian for its larvae. The wasp first lays its eggs within its host, a caterpillar of the moth species Thyrinteina leucocerae, after which they will develop until they are ready to pupate. This is where the weirdness begins. The larvae exit the host peacefully and create cocoons nearby, while the still living caterpillar starts to exhibit highly unusual behavior. The caterpillar stops feeding and ceases all movement, only springing to action when an intruder draws near, when it will suddenly vigorously thrash its head back and forth. This is done at the slightest provocation or disturbance, and serves to discourage any potential predators from feeding on the pupating wasp larvae, essentially serving as their own personal bodyguard until they emerge from their cocoons, after which the caterpillar dies.

One parasite that is known to infect humans and which can also alter behavior in animals is Toxoplasma gondii, which is usually found in rodents such as rats and small birds. Animals infected with the parasite will lose all fear of their natural enemies, and rather than flee from predators such as cats will feel compelled to run towards them. This of course leads to the rat or bird being eaten and the parasite carrying out the rest of its life cycle through the cat and its feces. It is unclear just what effects humans have when infected with Toxoplasma gondii, and there has been debate that it causes everything from no effect at all to major behavioral aberrations and schizophrenia, but it is at this point unknown. Still, it seems possible that an engineered form of such a parasite could lead to something very much like a zombie.

None of these cases we have looked at here involves the process of actually reanimating dead bodies to run amok, but they certainly illustrate the potential for a very real zombie apocalypse that would probably be just as bad. This will probably never happen, at least let’s hope not, but it is interesting to look at the very real scenarios for something out of the movies occurring fore real, and to look at the chilling possibilities that point to the chance of this happening, no matter how slim it may be. We tend to think of zombies as purely fictional constructs, but it seems that there is a very real basis for being at least unsettled by the off-chance that this may happen for real.