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The Real Science Behind Zombies In Video Games

A Ph.D. in Biology shares his thoughts on zombies in the wild and whether bacteria and viruses are correctly shown in video games

Editor’s note: Alexander Panchin is a Ph.D. in Computational Biology and a Senior Researcher at the Institute for Information Transmission Problems of the Russian Academy of Sciences. In his works, Alexander covers the themes of biotechnologies, genetic engineering and argues the harm of GMO.

Here, Alexander discusses various zombie types and their origins as well as how correctly they are presented in popular game series.

We can see diffferent zombies in video games. In fantasy worlds (Baldur’s Gate, Diablo, Divinity: Original Sin), the dead are resurrected by witchcraft, demons and necromancers. In Half-Life, victims of extraterrestrial parasites – headcrabs – become zombies. In Resident Evil, the Umbrella corporation creates artificial viruses, while in Plague Inc. humans are enslaved by Neurax brainworms. In The Last of Us, the zombie apocalypse is caused by parasites related to the Cordyceps fungus. In Left 4 Dead, the world is plagued by the virus similar to rabies. Reapers with synthetic intelligence in Mass Effect are engaged in indoctrination – they enslave organic forms of life by using special signals.

Is it plausible from the scientific point of view?

Magic Zombies

Haitian folklore has wizards – bokors – who can resurrect humans from the dead. As a result, they get zombies – the servants of their creators who are deprived of their own will. In Haitian criminal code, zombification is equated with murder. It is believed that a person who is poisoned by a special potion or buried alive turns into a zombie.

An anthropologist Wade Davis hypothesized that tetrodotoxin is the main component of bokors’ poison. It is obtained from fish related to fugu. Tetrodotoxin paralyzes the victim so the poisoned person may be mistaken for a dead man and buried – that’s exactly what bokors need.

According to Davis, there are two possible outcomes: many victims simply die because they can’t breathe anymore but some survive until bokors open their grave. The fate of these survivors is bad: they are beaten, fed with stramonium (a plant that causes severe hallucinations) and are enslaved.

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Posters for the movie White Zombie in Variety, 1932. An extract about zombification from Haitian criminal code is placed in the lower left corner of each poster

There is not much information about such ‘’zombies.’’ In 1997, the Lancet medical journal published an article about the results of the investigation of three cases when Haitians stated that they had met a deceased member of a family who was ‘’resurrected from the dead.’’ In two cases, DNA analysis did not confirm the alleged kinship – an additional reason not to believe the testimony on miraculous resurrection.

Only one case looked probable: the woman was identified by various villagers while her grave was empty. The ‘’zombie’’ lacked any motivation, was not able to talk and to feed herself. She was diagnosed with catatonic schizophrenia.

Alas, the true undead are not as interesting as they appear in fantasy. However, it may turn out that in the games we are beating people who are in need of medical assistance rather than walking dead.

Alien Parasites

The interaction between the parasite and the host is the result of their long-term evolution. Learning to manipulate the owner’s behavior is a very advanced skill, and not all parasites have acquired it. However, even those who learned how to turn their hosts into zombies are quite selective in their choice of victims.

Some types of parasitic wasps turn caterpillars into zombies, others choose spiders as a target. In the first case, the wasps’ maggots eat the caterpillar from the inside and then exit its body to pupate. The ‘’zombie caterpillar’’ remains alive for some time and protects the pupae, driving away ants and other potential predators, and sometimes covers the pupae with its own silk. It’s a bit different with zombie spiders: the wasps’ maggots force them to weave very strong cobwebs that are not intended to catch flies. It is not likely that the above-mentioned parasites could have easily found new hosts with another structure of the nervous system.

The alien parasite has no time to adapt to the life inside the host from the Earth. The human brain is a complete mystery for such a parasite.

Problems can arise not only during the attempt to take control of the nervous system, but even during the attempt to simply eat a representative of an unfamiliar species. Harmless human metabolites can be a poison for the alien, while the harmless microorganisms that live with humans can turn into deadly pathogens (as in the War of the Worlds by Herbert Wells).

If aliens or residents of another dimension have the biological ability to turn people into zombies, we must suggest that this is not their first contact with mammals or that they were designed by someone.

In the case of headcrabs from Half-Life, we have two potential explanations. The first one is that headcrabs destroy cerebral hemispheres of the victim but leave the spinal cord intact. Back in the sixties, scientists from the Moscow State University’s Research Institute of Physicochemical Biology named after A.N. Belozersky and the Institute of Information Transmission Problems have discovered a locomotor center in the brain and have shown that the brain does not control each individual motoneuron of the spinal cord but rather activates the embedded neural networks. This is similar to driving a car: you do not control each cylinder but simply push the gas pedal.

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Thus, the headcrab could have figured through trial and error how to send an electric signal in the spinal cord and make the human stand, move forward or wave his hands (found the above-mentioned ‘’gas pedal’’). At the same time, cases involving zombies who walk without any training are unclear.

The second explanation differs from the canonical beliefs. What if headcrabs are not parasites but predators who do not turn people into zombies? They grow their own legs, arms, nervous system, and just use the human body as a frame and a source of building material. The whole set of changes in the body of the zombie, whose hands became like pincers, speaks in favor of this theory.

Mutant Viruses

The Green Flu is the virus that caused a massive zombie epidemic in the game Left 4 Dead. It is believed that it is a mutated rabies virus. An ordinary rabies is transmitted via bites. Once in the wound with the infected saliva, the virus moves along the nerve processes into the central nervous system and then to the salivary glands. The virus causes aggression, progressive encephalitis, death of the nerve cells and the subsequent death of the host. The rabies needs to make a step forward to become like the Green Flu – it needs to spread more effectively and cause more changes in the hosts’ behavior.

The rabies virus is close to Ebola, measles, mumps and many other known pathogens, which have a similar genetic material (single-stranded RNA molecule) and a number of life cycle features.

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In contrast to rabies, measles is a respiratory infection and is highly contagious. According to the World Health Organization, there were 134,200 cases of death from measles worldwide in 2015. In 2000 – 2015, the vaccination against measles prevented about 20 million deaths.

Let’s look at the following scenario: the rabies virus has exchanged genetic material with measles or some other similar virus that has a more effective transmission route. This exchange is called reassortment and is a known phenomenon which is possible when two different viruses infect one cell. The resulting pathogens can be quite dangerous, like in the case of the swine flu.

Reassortment could explain the mass infection in Left 4 Dead. It is highly likely that the Green Flu is transmitted not only through bites. The existence of boomers – the giant zombies who vomit (right on the survivors!) with a liquid that looks like a concentrated snot or slime speaks in favor of this theory. By the way, measles causes a rash which may be the reason why the skin of zombies is so unattractive.

It is not hard to explain the phenomenon of survivors, people who are resistant to the virus. First, in order to penetrate the cell, viruses need to interact with certain protein molecules on the cell’s surface. Some people, due to genetic characteristics, may not have typical molecules. For example, there are people who are resistant to the human immunodeficiency virus due to mutation in the gene which encodes the CCR5 protein. Normally, HIV binds to this protein (approximately 10% of people have the mutation in one copy of the gene, but the full protection requires changes in two copies of the gene and occurs in one human out of a hundred).

Second, it is possible that humans who have been earlier vaccinated against ordinary rabies (for example, because they were bitten by a dog) have retained the immunity and are not exposed to Green Flu. These are not mass vaccinations, which could explain the small number of survivors.

As for the aggression of the zombies in the world of Left 4 Dead, it is possible that is caused not only by the virus-related changes. The scale of the epidemic points to the collapse of the healthcare system. If active opponents of vaccines come into power, they may cause such a catastrophe. If you look at how some of them behave on the Internet, you may get the impression that you do not need any rabies to provoke aggression in them. The infected have likely reached the ‘’boiling point’’ of denying the effectiveness of the vaccines or even denying the existence of the Green Flu virus itself so they attack survivors who are simple proponents of evidence-based healthcare (Note: each of the survivors has their own first aid kit!).

Artificial Viruses

Viruses are characterized by a shell and its contents – the genetic material of the pathogen and auxiliary molecules. Today, artificial viruses are created by scientists for peaceful reasons, for example, for gene therapy. You can take a shell from a dangerous virus that can infect a certain type of cells but replace the genetic material to something harmless or even useful. For example, if we want to cure hemophilia, a disease that causes poor blood coagulability, we can transport the correct copies of the genes that impact blood coagulability right in the cells of human liver.

Theoretically, nothing stops you from combining viruses for bad goals, taking genes from different viruses, combining them and testing them with different shells to achieve the optimal ratio of infectivity and lethality while maintaining the necessary symptoms. However, it’s easier to use natural solutions for practical tasks. Earlier, we discussed that the combining of the virus material occurs in nature. However, a man can do the same in the lab. In this sense, viruses from Resident Evil may be very similar to the Green Flu in Left 4 Dead, with the only difference being that you can honestly put a ‘’GMO-free'’ label on the sample of the boomer snot.

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The only problem of the viruses from Resident Evil is the excessive survivability of some of their hosts. No virus can make the host immortal. It can suppress pain, but no more.

Zombie Fungi

The idea for zombies from The Last of Us is based on scientific knowledge on fungi that can infect arthropods. The cordyceps forces infected ants to climb higher and choose optimal conditions regarding humidity and temperature. Under favorable conditions, the fungus grows through the head of the victim, and its spores are spread by the wind. Can this fungus infect a human?

There are some parasites that are known to infect arthropods and humans. Ticks transmit the virus of encephalitis (which, by the way, penetrates into the human brain) or bacteria that cause borreliosis. Lice transmit typhus, fleas carry plague, while mosquitoes are known for the malarial parasite. However, we should note that carriers of these diseases have different problems because of them in comparison to humans.

Fungal infections that are transmitted from human to human are also known. The US Center for Disease Control is currently concerned with cases of infection by the fungus Candida auris. It is able to spread through bloodstream, infecting not only the skin but also internal organs including the brain. As in The Last of Us, the fungus can affect the eyes. Also, the pathogen is resistant to a wide range of antifungal drugs and is very dangerous with a 60% mortality rate.

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All in all, we can say that the main idea of the infection in The Last of Us does not contradict scientific facts. Also, the mechanism of echolocation used by zombies with eaten eyes is interesting. Indeed, some blind people learn to navigate in space by clicking their tongue. On TED Talks, you can find a performance by Daniel Kish who demonstrates this skill. In nature, some bats are using it.

Brain Worms

There was a reason why the creator of the biological simulator Plague Inc. was invited to the American Center for Disease Control. There, he shared his experience of popularizing epidemiology with specialists. The game shows a lot of interesting things about the ways of infection transmission and about the methods of fighting against them.

The player’s task is to eliminate humanity with the help of bacteria, virus or another pathogen. If any members of our species survive in the end, you lose.

Over time, the player gains access to new types of pathogens including brain worms Neurax which affect the host’s behavior. For example, they can force their hosts to board a plane and go to the necessary country where the infection has not yet appeared.

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The ability of some worms to penetrate the brain is well documented.  The larval stage of the armed tapeworm can penetrate the central nervous system causing neurocysticercosis. Epilepsy is the main and, typically, the only symptom of this disease. While not all patients with neurocysticercosis have epileptic seizures, in some parts of the world neurocysticercosis is one of the leading causes of epilepsy.

Humans often have ‘’religious enlightenment’’ during, after and between epileptic seizures. The game hints at this scenario as one of the possible finals is the humanity’s submission to worms that are being worshiped like gods.


In human society, ideas are spread faster than parasites. Some of these ideas may be parasites in a sense that they force people to sacrifice their interests for the sake of their spread with examples being missionary work and the desire of some people to talk about God with you.

If viruses use cells to spread themselves, ideas are our brains. Often, text and words are more effective to change someone’s behavior than attempts to impact the brain with chemicals, parasites or neuro implants.

In Mass Effect, organic creatures that are exposed to contact with the Reapers – the machines of mass genocide powered by artificial intelligence – slowly go mad or often turn to the side of their enemies.

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The secret of indoctrination is not revealed in the game. However, we have one clue: the victim develops paranoia, the feeling that they are being watched, and the supernatural ideas about the Reapers. Carrie Poppy, an investigator of the paranormal, experienced similar feelings without being exposed to the Reapers.  Her lecture is available at TED. She believed that a ghost lived in her house and even performed religious rituals to get rid of it – with no success.

After this, she turned to ‘’ghost hunters’’ of a special character – to the skeptics. They advised her to call gas workers in order to check the content of carbon monoxide in the house. A poisoning by this substance can cause the above-mentioned symptoms. The employees of the gas service told her that she could have died if she did not call them – the carbon monoxide content in her house was well above the norm.

So, before you side with the voices in your head, be it angels, Reapers or ghosts, you should exclude more plausible explanations like a gas leak in the house or on a spaceship. However, Reapers also used neuro implants for manipulations, and their impact cannot be explained by gas.


In Deus Ex, you can find special viruses of artificial nature – nanites. Nanites – basically, nanobots – can be used to improve organs and remove damaged cells, opening the path to immortality. However, Bob Page secretly used them to organize a global pandemic and at the same time provided the medicine – Ambrosia.

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Nanite attached to human tissue. Technology from the Deus Ex series

In the real world, the perspectives of nanotechnology become increasingly evident. For example, it is possible to make nano-boxes from DNA that will contain medicine that opens during contact with cells of the desired type. In 2016, the Noble Prize in Chemistry was given for the design and production of molecular machines, very small motors that are capable of carrying a certain type of motion. While we are not talking about the creation of Deus Ex – like nanites, we get closer to such technologies. By 2052 (the events of the first Deus Ex), the useful nanobots could become a reality.

To sum it up, I would not count on immortality as a walking dead man. However, I’d hope to extend life with the help of biological viruses like the ones used in gene therapy.

Ostrog November 8, 2018