Below is a faux research article I wrote on ‘zombification’ in humans for a course analyzing the biological, political, sociological, and psychological aspects and implications of a zombie apocalypse.


Zombified humans display an array of behavioral traits which differ greatly from non-zombified humans; these traits are manifest in the relentless aggression and insatiable hunger for human flesh which are characteristic of zombie behavior. The danger which these zombies present to humanity is enormous, so understanding the sources and processes of zombification is vital to combating the zombies and preventing further cases of zombification. There are a number of possible pathogenic sources of this zombification, but one of them, parasitic zombification, is arguably the most likely. Some parasite species have demonstrated an ability to alter the behavior of their hosts, and the processes through which they do this are beginning to become better understood. As this parasitic behavioral manipulation comes closest to the sort of behavioral changes we observe in the zombies, detailing the processes of parasitism and host manipulation and applying them to a zombie model is very important. Doing so will greatly aid scientists and government officials in understanding, combating, and preventing this zombie apocalypse.

Background on Parasitism

Parasitism entails a non-mutual relationship between two organisms in which one, the parasite, benefits at the expense of the other, the host (Chandler, 1955). These parasites can be protozoa, which are microscopic, one celled organisms that live and multiple within their host;  helminths, which are large, multicellular organisms which also live within their host but cannot multiply while in an adult stage whilst inside their host; and ectoparasites, which include ticks, fleas, lice, and other organisms which attach or borrow into the skin of their host and remain there for long periods of time. Parasites often show a high degree of specialization, reproduce at rates which are much faster than their host, and rely upon their host in order to propagate their species (About Parasites, 2010).

Parasites can come in an array of shapes and sizes, infect and attack a diverse range of species and bodily systems, and cause ailments whose symptoms can be inconvenient to lethal, yet one specific group of parasites are particularly dangerous and, in the case of human zombification, important for study. These parasites hijack their host’s nervous system and manipulate their behaviors, often producing zombie-like behaviors in their host (Knight 2013). The behaviors which parasitic host manipulation can cause have often been divided into three well documented categories (Poulin et al., 1994). In the first, parasites manipulate their hosts in such a way as to favor transmission to their next host, often by rendering the former more susceptible to predation. In the second, they can  force a host into a habitat other than the one in which it usually lives, because the parasite must either exit or propagate in that habitat. The third type of manipulation causes the host to attempt to transmit the parasite via the spread of blood. This sort of transmission is most often seen in blood-sucking insects, who transmit the parasites while gorging on the blood of their prey.

Mechanisms Used by Manipulative Parasites

Manipulative parasites use a number of different mechanisms to control the behaviors of their hosts. Different parasites make use different mechanisms, producing different behaviors and effects, yet the array of possible mechanisms and results make it clear that parasites have a large set of tools for controlling and manipulating their hosts. These mechanisms include energetic drain, site of infection influences, manipulation of the immune system, and neuromodulation.

image00Rates of behavioral modification mechanisms used by manipulative parasites

One way Parasites manipulate host behaviors is through the extraction of energy, in the form of nutrition, from their hosts. When the host becomes starved for nutrients, their behavior might change, and if the energy drain impairs their physiology they might become sluggish or display lower physical performance. On the other hand, if a host is drained of energy, it might become more active and increaser foraging rates. Either way, the change in behavior could benefit the parasite responsible for the energy drain. Yet while this method of behavioral alteration demonstrates that such manipulation need not be complex, it does also lead to general behavior changes that might not benefit the transmission of the parasite, leading to inefficient transmission rates and putting the host at danger of malnutrition and death, thereby also killing the parasite (Lafferty & Shaw, 2013).

Another manner in which parasites manipulate their hosts is through the use of their host’s immune system. Because parasitic infection elicits host immune responses that are designed to overcome the invading parasite, parasites have to negotiate the host’s immune defenses in order to establish an infection. The continuing nature of the infection means that the parasite must continually evade the host’s immune system. However, many parasites demonstrate the ability to exploit host immune defense mechanisms for their own benefit. Research has shown that  parasite-exerted effects on the immune system may influence neuromodulator pathways, thereby altering the behavior of the host. For example, neuroinflammation is a common immune response of the brain to injury or invading pathogens. Several studies have demonstrated that that parasites may incorporate these host neuroinflammatory responses into their behavior modification strategy, using things such as nitric oxide (NO), rodlet cells, and alteration of neuromodulators to alter host behavior. For example, nitric Oxide, in addition to an immune response, functions as a neurotransmitter that can influence brain monoaminergic activity (Helluy & Thomas, 2010). Another example is the trematode Schistoso mamansoni, which secretes opioid peptides into its host, thus influencing both host immunity and neural function (Kavaliers et al., 1999). Inhibiting the host immune system while also utilizing it to neurally control the behavior of the host  is an evolutionary capability which has given manipulative parasites great advantages over their hosts. As parasites are constantly being target by the hosts immune system, they will consistently have resources to alter the behavior of the host.

Manipulative parasites can occupy a range of sites within their host. They are often found in the body cavity, muscles, central nervous system, and other parts, such as the brain. By occupying and damaging key these organ systems, these parasites can change host behavior. For example, the parasite Diplostomum spatheceum infects and damages the eye of a fish, causing it to become more susceptible to predation and thus the parasite more easily spread. Other parasites infect other parts of their host’s body, such as their muscles, in order to change the strength and function of these parts of the body and thus help the parasite spread more easily through transmission.

image01Infection location rates of manipulative paraistes in their hosts.

A key organ system for the manipulation of host behavior is the central nervous system; these parasites are able to manipulate behavior either through damage of the system or more subtle, neural manipulation (McConkey et al., 2013). The location in the brain and central nervous system of where this manipulation occurs is significant: various receptions of a certain hormone or neurotransmitter may produce different behavioral results depending on where they are located in the brain (McConkey et al., 2013). Parasites which want to manipulate their hosts behaviors in specific ways via neuromodulation must thus be located in and targeting specific locations within their hosts brains.

Numerous studies have shown that parasites achieve manipulation by directly or indirectly altering concentrations of hormones or neurotransmitters in their hosts. Concentrations of these substances that can be synthesized by the host and that have effects on behavior are altered following infection, thereby changing the behavior of the host. These substances could be produced by the parasite and released into the host, or actively taken from the host by the parasite and then rereleased (Knight 2013).

Monoamine neurotransmitters, which include the catecholamines DA, adrenaline (epinephrine, EP) and NE, the indoleamine 5-HT, and octopamine (OA), are potent neuromodulators. Monoamines influence many types of behaviors in vertebrates and invertebrates, including those related to activity, movement, stress, social activity and reproduction. The manipulation of these neurotransmitters is common in parasites which control their host’s behavior. One of the most striking and consistently documented behavioral modifications can be seen through parasite-induced changes in indoleamine 5-HT. Changes in 5-HT-altered behaviors in both vertebrates and invertebrates demonstrate the ubiquity of 5-HT as well as its widespread use in animals of varying complexity. Indeed, 5-HT is a major neuromuscular neurotransmitter used in the most primitive animals and its role expands into neuromodulator and neurohormone in more complex animals, where it controls additional physiological functions and behaviors, including stress responses and immune challenges. By altering indoleamine 5-HT, parasites take advantage of their hosts through the manipulation of their behaviors (Knight 2013).

The other neurotransmitters also have significant impacts on host behavior. DA has been found to stimulate locomotion, aggression, dominance and reproductive behavior in fish and mammals, and when altered can produced altered behavior in these categories. OA is a neurotransmitter and hormone that regulates other neuromodulators and that also influences many behaviors, including fight-or-flight reactions, stress, aggression, locomotion and feeding (Knight 2013).

Despite an understanding of these mechanisms for altering host behavior, researchers still do not know whether parasites alter host neuromodulators by secreting their own neurochemicals to produce a change in host neurochemical activity, or whether they secret chemicals which trigger a host response, thereby leading to altered neurochemical activity (Adamo 2013). Unfortunately, the complexity and interconnectedness of neuromodulator systems make it difficult for researchers to discern the specific mechanisms responsible for changes in behavior. Yet the complex nature of these systems might also help parasites induce widespread behavioral changes in their hosts with minimal effort; because these monoamines regulate one another in feedback loops, a parasite need only alter the activity of one in order to effect regulatory changes in the others (Knight 2013).

Applying parasitic host manipulation to the zombie model

Zombified humans display an array of behavioral traits which are starkly different from non-zombified humans. They are relentlessly aggressive, and have an insatiable desire for human flesh; indeed, it seems that the main motivating force for zombies is the search for and consumption of human flesh. They also display significantly lessened cognitive skills and capabilities, demonstrating a change in the physiology of their brain. As of yet, the source of zombification has not been identified, yet it is known to be highly infectious and spread through the bite of a zombie or the transference of zombie blood (Mogk 2011).

Could zombification be caused by a parasite manipulating human behavior? The characteristics of zombies, zombification, and parasitic behavioral manipulation seem to point in that direction. Zombification occurs from the transfer of blood and from a bite of a zombie. It is entirely possible that a parasite causing the zombie infection is transferred through blood and that, by attacking humans and spreading their blood, zombies are acting as a sort of vector for these parasites. If this is the case, the parasite would fall into the category of host manipulator which the host to attempt to transmit the parasite via the spread of blood. While this sort of host manipulation is most often seen in host insects, it could be that this parasite is making use of humans instead.

The insatiable hunger which zombified humans display could be a result of the energy drain caused by the parasite as it feeds off of and controls the human host. By reducing the amount of nutrients in the host body, the parasite could be causing the host’s hypothalamus to increase the host’s desire for food in order to replenish the nutrient loss. By doing so, the zombie will continue to display and act upon its hunger for human flesh, thereby causing it to continue to attack humans and spread the parasite.

The lessened cognitive capabilities of the zombified humans could be a result of damage caused to the frontal lobe by the parasite. Such damage would decrease thinking function, memory tasks, planning, and attention; all of these traits seem to be deficient in zombified humans (Mogk 2011). This damage could perhaps be intentional, for the aggressive behaviors which zombies display against other humans require a lessened sense of empathy, reasoning skills, and other traits which inhibit these behaviors in nonzombified human beings.

The behaviors which zombies exhibit could be the result of behavioral modification caused by parasitic manipulation. As has been discussed, the monoamine neurotransmitters DA and OA influence locomotive, aggressive, and dominant behaviors in mammals. It is entirely possible that the parasite infecting zombies manipulates these neurotransmitters to increase aggression and dominance in zombified humans, thereby causing them the rage which fuels their attacks on humans and the transmission of the parasite. Furthermore, it is likely that this parasite manipulates other neurotransmitters and hormones to inhibit the sense of pain which zombies may feel, thereby causing their seeming immunity to pain, and increase their desire for food in the form of human flesh. Further neuromodulation, albeit in ways not yet discussed or researched, can cause the zombies to specifically seek out non-zombified human beings for attack and prioritize humans as a source of food, ensuring that the parasite thus has the greatest rate of propagation possible.

Further research

The recent spate of zombie attacks have elicited much notice and concern, yet little study. Although the characteristics of the zombies are understood and the way in which the infection is spread is recognized, the specific source of zombification and the method in which humans are zombified has still not yet been discovered. In order to effectively combat the zombies and prevent a disaster on an apocalyptic scale, scientists and government officials must discover this source and study it in order to develop an effective medicine and countermeasures.

This article has outlined the possibility for the source of zombification to be a behaviorally manipulative parasite. The characteristics of the zombies all fit within the scope of symptoms caused by such parasites, and the behaviors of the zombies seem to resemble the behaviors which such parasites could cause. By proposing a possible source of zombification, this article hopes to help scientists more easily and readily pinpoint, or discount, a possible cause to the looming zombie apocalypse. In doing so, they will be more prepared to quickly develop countermeasures which will help halt the spread of the zombies and the zombie infection.

Scientists must now make it a priority to capture a zombie and study its physiology, in particular its brain. They should focus on determining whether there are any parasites located within the brain of the zombified human, and, if so, begin the process of developing an effective medicine or vaccine to combat and prevent the spread of these parasites and their behavioral manipulation. The specific mechanisms and neuromodulators and neurotransmitters which parasite uses, if it is indeed found, will also be an area of study which is necessary. By studying these, scientists will better understand how parasites might control their hosts, how future parasites might attempt to control a human host, and how the human brain works with regards to these functions anyway. Such research will be invaluable for preventing future zombie outbreaks and for better understanding the anatomy and function of the human mind.


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