May is “Zombie awareness month”. To mark the occasion, Noam Leviatan reviews the history of the living dead, the science of real and imaginary zombies, the link between zombies and public health, and ways to increase your chances of surviving a zombie apocalypse

Zombies have been a part of popular culture for many years. But over the last few decades, their popularity has soared and now they can be seen everywhere—from computer games to television series, movies, books, comics, and in the streets, on “zombie walks. In fact, they have gained so much popularity that May has been declared Zombie Awareness Month.

The awareness campaign, though only seemingly somber, was initiated in 2007 by the Zombie Research Society, which chose its particular timing for the numerous zombie films, such as The Night of the Living Dead, set in May. The campaign’s declared goal is to increase awareness of the living dead and the inevitable zombie pandemic. And if people would also learn how to prepare and cope with other, real-world disasters, well—that’s a bonus.

According to the Zombie Research Society, a zombie is “a relentlessly aggressive, reanimated human corpse driven by a biological infection.” Zombies harbor an insatiable hunger for human flesh and their bite can transform others into zombies. These are the modern zombies that typically come to mind when we hear the Z-word and which are portrayed on television and movies, including World War Z, The Walking Dead, Zombieland, and Pride and Prejudice and Zombies. However, when zombies made their first appearance in western culture, they were quite different.


Reanimated corpses became a part of western culture. An image from Night of the Living Dead. Photo: Common property.
Reanimated corpses became a part of western culture. An image from Night of the Living Dead. Photo: Common property.

Dawn of the living dead

It is commonly accepted that the word “zombie” first entered the English language in 1819 in the book History of Brazil by poet Robert Southey, who also published Goldilocks and the Three Bears. But there is evidence that the word was introduced 14 years earlier. Southey so loved the word that he named his cat “Zombie”.

In his book, Southey wrote about Zumbi dos Palmares, the African-Brazilian leader of a republic of escaped slaves in Brazil. His name originates in West Africa, and is derived from zumbi – which means fetish, or a magical object, and nzambi – which means soul or deity in the languages of the Congo, Angola, and the Democratic Republic of Congo. These words made their way to the Caribbean Islands along with the slaves that were forcefully taken there from Africa.

In the middle of the 17th century, France obtained lands in the western part of Hispaniola Island in the Caribbean, which would later be known as Haiti. The French brought African slaves with them, to work in the sugar cane plantations and coffee fields, converting them to Catholicism and working them to death. In order to counterbalance the thousands of deaths, the French abducted tens of thousands of Africans every year and transported them as slaves to Haiti. Slave trade made the island one of the French empire’s richest colonies. By the end of the 18th century, 60 percent of the coffee consumed in Europe and more sugar than that produced in all of the Caribbean British colonies combined was produced in its small area. During that period, the number of slaves in Haiti reached around 700,000, most of them African, supervised by some 30,000 white settlers.

A slave rebellion broke out in Haiti in 1791. It lasted 12 years, during which the rebels defeated the military forces sent by Napoleon Bonaparte. The sovereign state of Haiti was established in 1804, America’s second independent nation, after the United States.

The abducted African slaves were brought to Haiti from different places, bringing with them different religions, which they did not relinquish despite their forced conversion to Christianity. The emerging Haitian religion was a combination of Catholic Christianity, western Africa religions, and the natives’ beliefs. Thanks to Haiti’s independence, this religion wasn’t repressed and developed into the system of beliefs and a religion that are today called Voodoo.

The zombies developed in popular beliefs derived from Voodoo and from the violent period of slavery in Haiti, from which one could escape only through death. The zombies of Haiti are soulless corpses raised from the dead by a sorcerer, or a “bokor,” to serve as slaves without free will. This is the nightmare of any former slave: To rise, undead as a slave for eternity.


Zombies in a painting from the book The Magic Island, common property
Beliefs that were born of local traditions, slavery, and the Voodoo religion. Zombies in a painting from the book The Magic Island, common property

This is a magical place

Zombies entered popular culture when the United States occupied Haiti, from 1915 to 1934. Americans who visited Haiti during the occupation returned with terrifying stories of Voodoo and zombies. One of them was William Seabrook, who brought zombies into popular American culture: An American journalist, an author of popular travel books, explorer, alcoholic, cannibal, a fan of bondage and domination, he was fascinated with mysticism. After his visit to Haiti, Seabrook published his book The Magic Island in 1929, describing his experiences from Voodoo ceremonies he had participated in and the stories that he had heard from the locals. In one of the chapters, he even claims to have met zombies working in the sugar cane fields.

The zombies in the stories wore worn-out clothing. They walked heavily, shambling and weaving like a man in shock, or a drunk. The worst thing about them was their eyes. They looked like the eyes of a corpse. They were not blind, but stared blankly, like cattle, with an unfocused gaze and were unaware of what occurred around them. They were unresponsive if addressed and, if they made any noise at all, it was unintelligible. All the zombies did was to work in the fields at their master’s command, who beat them if they worked too slowly or if he thought they were being lazy.

They were forbidden to eat meat or food seasoned with salt, since a zombie who ate such things would regain its memories and ability to speak. When this happens, the zombies return to their village or place of burial, and die. Sometimes, before they return to the burial site, the zombies are free to take revenge on the bokor that raised them from the dead, and kill him. Aside from eating meat or salt, the death of the bokor that raised them also releases the zombies to their deaths – as did the death of the White Walkers (“the Others”) or the Night King in the Game of Thrones series, or in the movie Hocus Pocus.

Seabrook was initially alarmed by the zombie stories that he heard and met, but he later declared that there was nothing supernatural about them. These were not undead but “regular people, miserable and mentally wounded, idiots, who are forced to slave away in the fields.”

In February 1932, only three years after The Magic Island was published, the book inspired the creation of the Broadway Zombie by playwright Kenneth Webb. The play, about a plantation owner in Haiti whose husband turned into a zombie, was a complete flop, and ended its run with only 21 shows. That same year, brothers Victor and Edward Halperin created the first zombie film in history, White Zombie, starring Bela Lugosi, who had, only a year earlier, starred as the famous vampire, Count Dracula. Lugosi’s character in White Zombie was an evil sorcerer who owned a sugar plantation in Haiti, whose workers and servants were zombies he had created.

The creators of White Zombie didn’t credit Seabrook, despite their use of texts and descriptions from the zombie chapter of his book; yet he did not sue them. However, Webb did sue the Halperin brothers on copyright grounds, but lost the trial, with the judge ruling that the term ‘zombie’ is public domain.

Rising interest in zombies led scientists and anthropologists to devote efforts to discovering the source of the Haitian zombie stories. In 1937, American author and anthropologist Zora Neale Hurston published Tell My Horse, a book in which she describes her research on Haitian traditions and beliefs. Hurston claimed that zombies were real, that she had met them, and that she had even taken a photograph of one in a village in Haiti. A family she had interviewed claimed the zombie was their relative Felicia Felix-Mentor, who had died in 1907, 30 years earlier, and who, in life, had suffered from a limp due to a broken leg. The unresponsive zombie similarly limped and walked slowly. Hurston report was ridiculed: The woman in the photographed she took, rather than being undead, was a handicapped woman who apparently also suffered from mental illness. One of the families from the village believed that she was Felix-Mentor that had returned from the dead and adopted her. An X-ray scan even showed that unlike Felix-Mentor, the photographed woman had never broken her leg.


 A scene from the movie White Zombie, common property.
The court ruled that the term zombie was public domain and there can be no copyright attached to it. A scene from the movie White Zombie, common property.

Instant zombie powder

As the years passed, no realistic basis for zombies emerged. In several cases, people allegedly returned to their villages after death as zombies, and others claimed to have been zombies under the control of a bokor and released upon the bokor’s death. Some of these were impersonators; others, like Felix-Mentor, were people who suffered from mental illnesses; and others still, at times also ill, truly and innocently believed that they were zombies. All that changed, allegedly, in 1980, when a man named Clairvius Narcisse marched into a village in the center of Haiti, approaching a woman and telling her that he was her deceased brother who had been buried 18 years ago. The woman believed him, as he introduced himself using a nickname known only to her and her close relations and added details that only they knew.

Contrary to the previous “zombie” appearances, Narcisse’s death was documented. On the April 30, 1962, he arrived at the American hospital in Haiti suffering from a high fever and spitting blood. He died three days later. He claimed to have been aware of all that occurred, but was unable to move or talk.

According to his account, on the night of his burial, a bokor extracted him from his grave, brought him back to life as a zombie, and put him to work with other zombies in a sugar cane field in northern Haiti. Only when the bokor died, Narcisse and his fellow zombies were released, and he came looking for his family. However, he was unable to lead researchers to the field where he had allegedly worked as a slave.

Since Narcisse’s death had appeared in hospital records, the case caught the attention of a Haitian psychiatrist, who had tried for years to locate a “real” zombie to find the truth of how they were created. The psychiatrist believed that zombies were not dead people brought back to life, but were rather created from materials that make the living appear dead. Now that he believed he had evidence that there were real zombies, he was able to enlist the collaboration of researchers from the United States. He contacted Nathan Kline, a psychiatrist and a pioneer in the field of psycho-pharmacology, who studied the effect of medications on mental and behavioral processes. In 1982, Kline sent Wade Davis, a botanist and a biology doctoral student at Harvard University, to Haiti.

In Haiti, Davis attended spell-casting rituals and was deeply impressed by them, with no evidence of any critical thinking on his part. He met with several bokors and paid them undisclosed amounts, until finally he purchased eight “zombie powders” from bokors in different areas of Haiti. According to Davis, he paid 300 dollars for each powder. The powders varied, but seven of them contained the same four components: Traces of human, cane toads (Rhinella marina), Hispaniolan common tree frogs (Osteopilus dominicensis), and several species of pufferfish (Tetraodontidae) remains.

The bodies of pufferfish contain a toxin called tetrodotoxin. It is one of the world’s most toxic substances and is over 1000 times more toxic to humans than cyanide. Tetrodotoxin is present in the bodies of the fugu fish, a pufferfish served in Japanese restaurants; any mistakes on the chef’s part in cooking the fish can be fatal to the consumer. In Israel, too, there are occasional cases of poisoning due to accidental consumption of the lagocephalus, also known as the half-smooth golden pufferfish, which invaded the Mediterranean from the Red Sea through the Suez Canal.

According to Davis, the tetrodotoxin is the key to zombie creation. When bokors administer the toxin to their victims, it paralyzes them, slows down their heartbeat, and makes them appear to be dead. Several hours later, typically after burial, the victim recuperates and “rises from the dead,” alone or with help from the bokor. Since the victim was buried and grew up in a culture that believes in zombies, the new “living dead” believes that he or she has become a “zombie;” this is sometimes helped along by the bokor who administers sense-dulling drugs, such as the Datura plant (also known as the “zombie cucumber”) or beats the victim.

However, most researchers disagree with Davis, claiming that chemical analysis shows that the tetrodotoxin amount in the powders was negligible and cannot cause any effect. Furthermore, even if the toxin can cause a death-like state, it is highly unlikely that a bokor could give the exact dosage required without either actually killing the victim or conversely, causing no effect at all. Some researchers also claim that Davis had concealed findings that did not fit his claims.


From Night of the Living Dead, common property.
Those who grew up in a culture that believes in zombies will have an easier time believing that they themselves have become zombies. From Night of the Living Dead, common property.

Calming the nerves

Even if they did contain a significant amount of tetrodotoxin, zombie powders probably would not convincingly make a victim appear dead.

Tetrodotoxin’s activity affects sodium channels, which are protein channels found mostly in nerve and muscle cells. When these channels are open, they allow ions (charged atoms) of sodium into the cells, and their activity enables nerve signaling. Tetrodotoxin blocks the ions’ passage through the channels, thereby blocking the passage of the nervous signal and in effect preventing the muscles from contracting. This causes paralysis, and when the toxin reaches the respiratory muscles, it can lead to death. The toxin does not influence all of the body’s sodium channels in the same way, and its effect on the heart for example, is much weaker than on motor muscles.

When ingested, as in a Japanese restaurant for example, the toxin causes a prickling feeling on the lips, with complete loss of sensation in the lips within 30 minutes. `This is followed by salivation and often nausea, vomiting, and diarrhea. As the toxin advances, the loss of sensation spreads throughout the body, resulting in difficulties in walking and talking. At this point, the symptoms exacerbate: Paralysis sets in, the victim’s muscles relax, with difficulties in breathing that could cause death, weakening heartbeats, and loss of pupil response to light. If exposed to a large dose of the toxin, the victim could lose consciousness and cease breathing, dying if not ventilated. Those who don’t die recuperate after several hours, with no permanent damage.

Some of the symptoms could perhaps explain how a victim can become paralyzed, with a weak pulse and full consciousness, but these symptoms are not similar to the zombie state, not even to those described by Davis, who didn’t allude to flaccid paralysis. It appears that Haiti’s zombies are a fabrication, or cases of mistaken identity of people with mental disabilities or illnesses. Nevertheless, tetrodotoxin has other uses among mankind – analgesia of certain types of pain and treatment of heart problems.


The Diodon holocanthus fish. Source: Science Photo Library
The toxin blocks sodium channels in the motor nerves and paralyzes many muscles. The Diodon holocanthus fish. Source: Science Photo Library

They’re coming for you, Barbara

For several decades, the word zombie referred to living dead that serve a master, like the zombies of Haiti. Then came the modern zombie: A violent, aggressive, man-eating creature, which cannot be satisfied and can be killed only by decapitation.

Modern zombies first appeared in 1968 in George Andrew Romero’s Night of the Living Dead, a low-budget film that didn’t feature even a single zombie. Or at least not a creature that was called that. Romero’s inspiration came from several sources, chief among them was a 1954 book by Richard Matheson, called I am Legend. In the book, a bacterial worldwide plague turned all of humanity, except the hero, into violent, vampire-like creatures. Instead of zombies, Romero’s film features fresh corpses that rise from the dead, which, following the creative advice of his co-writer, John A. Russo, consume human flesh. When not pursuing a human (in a light jog), these living dead moved heavily and clumsily, like the zombies featured in White Zombie, another source of inspiration for Romero.

But Romero did not use the term ‘zombie’ to describe the creatures of Night of the Living Dead. Believing that he had created altogether new creatures, despite the ideas borrowed from previous zombie and vampire movies, Romero called them Ghouls, since he thought of zombies as the not-really-dead slaves of Haiti. After the film’s screening, the newspapers and the critics referred to its living dead as ‘zombies,’ and the name stuck; thus, in his 1978 sequel, Dawn of the Dead, Romero referred to the creatures as such.

The plot of Night of the Living Dead revolves around a woman, Barbara, and a man, Ben, who hide together with a group of people in a house in Pennsylvania from a swarm of bodies that have risen from the dead. Barbara arrives at the house and meets Ben after she and her brother were attacked at the cemetery by a strange man, from whom she alone managed to escape. Apparently, the bodies were reanimated due to radiation reflected from a NASA satellite returning from Venus, which turns any recently deceased corpse into a man-eating undead. But the film’s real conflicts are among the living humans, not between them and the cannibalistic monsters outside. The full movie:

Night of the Living Dead is one of the first movies to feature a black protagonist, and is the first horror film to cast a black actor in such a role. It is also the first to present zombies that aren’t controlled by an exterior master. Instead, they are reanimated corpses that devour humans, are infectious (or at least have a lethally infectious bite), and are more numerous than the humans. In addition, this is the first instance of showing a government trying to cope with the situation. Romero used the zombies as a metaphor, to criticize politics and society; in this case, the criticism is turned against an inefficient government, and implied against the war in Vietnam, showing that even under a zombie attack, the real threat to humanity are humans themselves. Formerly symbolizing the fear of slavery, the zombies of this film embody fears of the modern human society.

Life after death

A mistake on the part of Night of the Living Dead’s distributors led to its immediate release into public domain, facilitating a surge of films that borrowed heavily from its ideas – without running the risk of violating copyright laws. This situation encouraged the production of additional movies and the flourishing of the new horror sub-genre, “zombie films.” Throughout the changes introduced in the new zombie films, by Romero and others – from victims-turned-slaves using supernatural powers to human-flesh-hungry undead killers – nearly all the films used the modern zombies to criticize human society and expose man’s true nature.

In the 2000s, the zombie film sub-genre was revived, with the 2002 release of Resident Evil, the first in a long series of zombie films based on an eponymous popular computer game. In the computer game and the films, zombies emerged due to an infection with the ‘T virus,’ genetically engineered by the Umbrella Corporation, which can infect beings both living and dead.

In that same year, a low-budget zombie film called 28 Days Later reached movie theaters. The movie was a huge commercial success and received great acclaim, breathing new life into the genre. The film’s “zombies” were not undead, but very much alive – fast, violent, and full of rage. They owed their existence to animal-rights protesters who broke into a research lab in Cambridge, and, unheeding the warning of the lab’s scientists, released chimpanzees infected with the fictitious virus “rage.” The chimpanzees attacked their saviors, infecting them with the virus. The infected people were turned into conscious thought-lacking, raging “zombies” that were intent killing anyone who wasn’t infected with the virus, or to infect them. The virus spread rapidly throughout the British isle. The movie’s “zombies” lack all survival instinct, do not eat humans and, ultimately simply die of starvation. Like its predecessors, this zombie film criticizes human society, and, by showing uncurbed British soldiers, signals that “regular” humans are a danger graver than any zombie.

The commercial success of 28 Days Later led to an increased publicizing of zombie-related work: In 2004, a remake of Romero’s Dawn of the Dead and the first successful zombie comedy, Shaun of the Dead, were released. In 2005, Romero returned to creating zombie films and in 2009, another zombie comedy came out – Welcome to Zombieland, whose sequel will hit theaters later this year (another zombie comedy, The Dead Don’t Die was released earlier this year). The successful The Walking Dead television series premiered in 2010, based on a comic book series from 2003, with nine seasons to date. Even in Israel, two zombie movies were created – the first was a short comedy called Poisoned and the second, called Cannon Fodder, was a full-length movie about an elite IDF unit in Lebanon, under zombie attack. Popular zombie books include bestsellers World War Z, Max Brooks’ Zombie Survival Guide, and The Girl with all the Gifts by Mike Carey.

Trailer of The dead don’t die:

The attack of the disease-spreaders

Modern zombies aren’t created by magic or exposure to radiation or toxic gases, but by infectious diseases. Therefore, they can serve the epidemiology – the science of disease spreading in populations – as an allegory for the spread of infectious diseases. Likewise, they open a door for learning about disease agents in the real world and for discussing, while keeping a straight face, the epidemiology, treatment, and prevention of zombie infections. The majority of these infectious agents are viruses, but fungi, other parasites, and even prions, which are defective proteins that cause diseases such as mad cow disease, are also candidates.

In Resident Evil, for example, dead bodies infected with the T virus rise from the dead. Obviously, the infection of a corpse by a virus that reanimates it for its own purpose is completely fictional; the deceased do not return to life. Even so, an article published in 2017 conducted on mice and fish showed that some processes continue to function in the body even after its official death. The research showed that gene expression in different cells in the body continues to change up to four days after death. For example, there is an increase in the expression of genes related to inflammatory processes, fetal development, and cancer. Researchers believe that this occurs because the regulatory mechanisms, which inhibited these genes form being expressed in life, break down gradually following death, thus enabling suppressed genes to “come to life” for a short period. Understanding this “life after death” may help to improve organ transplantations, which can fail because of the activity of these genes, uncontrolled after the death of the donor. Likewise, if the body’s cells are still active after death, it is easier to accept the imaginary idea of some kind of parasite that can take control of a recently deceased body, supply it with energy and utilize it for its own needs.

This is the case with the virus that turns people into zombies in Max Brooks’ series of books. Once the virus penetrates the body, through a bite from an infected individual, an organ transplant, or other exchange of bodily fluids, the virus migrates to the brain, multiplies there and destroys most of it. It leads to the death of anyone infected and reanimates the host as a violent human-eating zombie. Nevertheless, the virus cannot infect a corpse, even a freshly deceased one. It must infect the body prior to death. Except for the reanimating-the-dead part and the speed of the disease’s outbreak, this imaginary virus is quite similar to the Rabies virus. Rabies can infect any mammal; it is secreted in the saliva and is mostly transmitted through the bite of an infected animal. It migrates from the bite wound along nerve cells until it reaches the brain, where it creates an acute inflammation, behavioral changes, and eventually causes death. One hundred percent of those infected die if they are not vaccinated shortly after they are exposed to the virus.

In The Walking Dead series, the whole world was unknowingly infected with the mysterious zombie-creating agent, even before the beginning of the series. Those infected are carriers, and the disease erupts and turns them into zombies – into the “walking dead” – only after they die, following an infection from a zombie’s bite or for other reasons. They return from the dead as long as their brain is intact, because, like the virus in Brooks’ books, the causative agent penetrates the brain and operates the most basic processes in it, which allow the zombies to walk around as a walking corpse.

In one of the series’ episodes, the survivors arrive at what is left from the American Center for Disease Control (CDC) in Atlanta. A researcher shows them a video of a brain scan taken upon the death of a man and his return as a walking dead. The researcher explains, “[The agent] penetrates into the brain similar to meningitis, the adrenal glands begin bleeding, the brain shuts down and then the vital organs… [The agent] restarts the brain stem. It makes them get up and start moving. The neocortex, the human part, it doesn’t go back on. The “you”. Its just a shell driven by instincts, without thought.”

It’s all in the head

The fact that head injuries result in changes in behavior is no longer surprising, and medications that affect the brain, electrical stimulation of specific brain areas or brain surgeries are used to treat different disorders.

A famous example of such a disorder is the peculiar case of Phineas Gage, a 25-year-old American railroad construction foreman who, in the mid-19th century, worked south of Cavendish, Vermont. On September 13, 1848, a blast that was meant to clear the way for the rail flung a meter-long iron rod at Gage. The rod went through his head and brain, piercing his frontal lobe. To everyone’s surprise, he survived and didn’t even lose consciousness. But Gage’s behavior after the injury changed drastically; his friends said that “he was no longer Gage” – from a responsible, calculated, meticulous, and normative man, he became childish, rude, grumpy, and irresponsible and was eventually fired. Today, we know that the frontal lobe is linked to impulse control and social behavior.

One night in 2009, Steven Schlozman, a children’s psychiatrist who teaches a course about the psychology of horror movies in Harvard, was watching Night of the Living Dead. In order to distract himself from thoughts about his wife’s recent breast cancer diagnosis, he tried to understand what kind of brain damage could lead to a Zombie-like behavior. He decided that zombies suffered from a syndrome he came up with: Ataxic Neurodegenerative Satiety Deficiency Syndrome; a complicated way of saying that zombies are clumsy, walk slowly, suffer from brain injury, and are constantly hungry.

According to Schlozman, zombies’ typical gait is due to injury in the cerebellum, or “little brain”, and the basal ganglia, areas in the brain that are critical for smooth and continuous motions. Parkinson’s disease, for example, is characterized by the degeneration of nerve cells in the basal ganglia. Like Gage, zombies’ frontal lobe is damaged, most of their cognitive ability is impaired, and they have no control over their impulses. As a result, the amygdala, a small structure in the brain that resembles an almond, affects their emotions endlessly, and increases their aggression and fury. The insatiable hunger stems from damage to parts of the hypothalamus, the part of the brain that is responsible for the sensations of hunger and satiety, such as what might happen in people suffering from Prader-Willi syndrome, which causes them to feel constantly hungry. And why do zombies moan and groan? Schlozman claims that with a diet such as theirs, they are constantly suffering from constipation.

Schlozman further elaborated about a possible cause for a zombie epidemic in his 2011 book The Zombie Autopsies: A new type of prion. Prions are proteins whose defective version can bind to other proteins of the same type and induce them to change their conformation as well, producing a chain reaction. When the defective protein accumulates in the brain, it kills nerve cells and the brain becomes spongy and perforated. The Kuru disease, for example, is a prion disease transmitted by eating infected human brains. The afflicted suffer from symptoms that sound familiar: Clumsy walking, decrease in cognitive ability, loss of speech ability and coordination, appearance of sores on their bodies, and finally, death.

In the film Zombieland, the zombie plague breaks out because of a version of a prion disease, “mad cow”. In reality, prions are not transmitted from one person to another without ingesting the infected protein, therefore there is no chance that they could create infectious zombies. To solve this problem, Schlozman invented in his book a prion combined with the influenza virus, thereby rendering it transmittable through the air. When Schlozman was interviewed by a radio station, they jokingly spoke of his book as if it was real, sparking slight panic in the public. Some of the listeners believed that there actually was a zombie-creating disease and were at a loss, reminiscent of the effect of Orson Welles’ 1938 broadcast War of the Worlds. It may sound strange that people believe such stories, but during the Ebola outbreak in 2014, there were stories on the internet claiming that the virus resurrected the dead and talking of the danger of a zombie apocalypse, and there was a real need to disproof these stories. Even nowadays, people who believe in such baseless things as the Earth’s being flat or that vaccines are dangerous are not unheard of.

The master within

In some zombie stories, the infectious zombification agent does not cause brain damage, but changes the behavior of its host, the infected human. The “rage” virus in 28 Days Later is an example of such an agent. In the computer game The Last of Us and the book The Girl with All the Gifts (which is highly recommended), a fictional mutant of the real Ophiocordyceps unilateralis fungal parasite is the cause of a zombie plague. Those infected by the fungus are controlled by it and become hungry, violent creatures, lose most of their cognitive functions, and feed mostly on humans. The longer the host is infected, the more changes it accumulates. These are live zombies, not dead ones, and even though there isn’t a bokor controlling them, they do have a master – the parasite growing within.

Contrary to the previous zombies mentioned, parasites taking control of other living creatures and turning them into zombie-like beings really exist in nature. However, none have been found that control humans – even the rabies virus doesn’t make the people infected with it walk around looking for other people to bite.

In our world, the O. unilateralis fungus infects ants, not humans, thus its unsurprising nickname “zombie ant fungus.” Its parasitic interaction with ants has a history of tens of millions of years and is known to science for more than a hundred years: It was discovered in 1859 by one of the fathers of evolution theory – Alfred Russel Wallace.

The fungus makes an infected ant walk strangely, zombie-like, away from the nest, then climb a plant and attach itself by its jaws to the bottom part of a leaf. The ant dies within about ten days, but the fungus is “prepared” for this: It dismantles the ant’s jaw muscles after it has attached to the bottom of the leaf, so that the ant remains stuck to the leaf even after it dies. Now the fungus can sprout new hyphae from the ant’s body, spreading infectious spores. The spores will penetrate the bodies of the ants running about the plant and sprout within them. The fungus then spreads throughout the ants’ bodies, and 16-25 days later, it sends them to their deaths in a similar fashion.

Until about a year and a half ago, researchers believed that the fungus penetrated into the brain of the ants in order to control them, but it turns out that fungal cells were spread throughout the ant’s entire body – head, chest, stomach, legs, and muscles – everywhere except the brain. The researchers who discovered this believe that the reason is that brain damage would kill the host before the parasite could reproduce and infect other ants. Instead, it controls the ants using different methods. Perhaps by directly controlling the ant’s muscles, the poor insect moves as a puppet on a string, its body not obeying its brain. Or – more likely – it controls the ant by secreting certain chemicals into the ant’s fully functioning brain. This is also the reason that it is unlikely that zombies would eat their victim’s brains, except for in cheap parodies. Zombies can’t exist without brains and victims that had their brains eaten can’t be resurrected as zombies.

The ant is evolutionarily distant from humans, but in nature there are examples of parasites that change the behavior of animals more closely related to us. One of these is Trematoda, a parasitic flatworm of the Killifish. When infected with Trematoda, these fish swim close to the water’s surface and occasionally jerk and surface, presenting their silver bellies – thus attracting predatory birds. Fish-eating water fowl are the parasite’s definitive host; that is, the Trematoda can sexually reproduce only in the birds. Uninfected fish tend to stay away from the water’s surface and are caught much less often than infected ones.

The infection cycle begins in water snails, which consume bird droppings carrying the flatworm’s eggs. The parasite then leaves the snails to find a fish, latches onto the gills of the killifish and makes its way into its brain. Once in the brain, the flatworm secretes chemicals that decrease the levels of the neurotransmitter serotonin and increase that of dopamine. This is apparently related to the change in the fish’s behavior.

An even more well-known example of a parasite that changes its host’s behavior is the single-cell parasite Toxoplasma gondii, which infects a variety of mammals and birds, and whose definitive hosts are cats. En route to the cat, the toxoplasma infects rats and mice, penetrates their brains and makes them permanently lose their ingrained fear of cats – and only cats. Thus, the cat gets an easy meal and toxoplasma to boot. The parasite reaches the cat’s intestines, where it develops to adulthood and sexually reproduces, producing thick-walled oocytes. These are secreted in the cat’s feces spread to soil, water, food, or anything potentially contaminated with the feces, until it is accidentally ingested by a new intermediary host.

Despite the stories, no connection has been found between the “crazy cat lady” syndrome and toxoplasmosis; the parasite does not affect the behavior of humans or their close relatives.


The fungal hyphae sprout from the ant’s head. Photography: Penn State University
An ant controlled by the deadly O. unilateralis parasite, hanging off a leaf. The fungal hyphae sprout from the ant’s head. Photography: Penn State University

How to survive a zombie attack

Even though zombies aren’t real, we can still learn from them. Schlozman’s Zombie Autopsies is a tool for learning about brain sciences. The CDC even referred to the syndrome that Schlozman concocted when, during the 2011 zombie awareness month, it used a zombie apocalypse as an opportunity to teach preparedness for emergencies and disasters. The publicity was so successful that the CDC’s website crashed due to an overload of visitors. A year later, the CDC published a list of dos and don’ts to survive in a world of “walking dead.” In addition, the CDC’s website contains comics and lesson layouts for zombie apocalypse preparedness. Even the United States Military and Department of Defense use zombies as a practice tool for coping with large unexpected disasters.

In case you were wondering, the CDC recommends to prepare an emergency kit that includes three and a half liters of water per day per person, canned food or other food that does not require special storage conditions, basic work tools, a radio and batteries, medications, a first aid kit, important documents, and so on. Instructions also include the predetermination of a safe convening zone in case of emergency, as well as an escape route free of threats. What the CDC’s list is missing is weapons, which may not come as a surprise, considering that the CDC’s real purpose is to prepare for emergencies more commonplace than zombies: Hurricanes, floods, or a run-of-the-mill epidemic.

In her blog, epidemiologist Keren Landsman added her own defense tips for facing zombies or other unknown diseases.

But if in spite of all of this, a zombie apocalypse does occur, mathematical models used in research on zombie-less plagues show that it can be survived and perhaps even thwarted.

In 2009, a group of mathematicians from Canada published a model that predicts the speed of zombie propagation in the population, showing which method the government should employed in order to best counter its propagation. The zombies chosen for this model were slow moving, human-devouring undead, which would therefore live forever if not destroyed. A person attacked (bitten) turns into a zombie only after 24 hours, and during that time, can infect others even before actually becoming a zombie. According to the model, if a single zombie reaches a city of half a million inhabitants, it would take eight days for all the inhabitants to turn into zombies and the human species to essentially go extinct. Attempts to quarantine the infected individuals or to treat them fail, and it appears that the zombies always win.

The only way to eliminate the zombies and save humanity is to destroy all of the zombies found, over and over again, as quickly as possible. After several such waves of zombie-killing, the zombies are gone and the inhabitants of the city can rest assured, at least, until the next zombie. A similar method eradicated smallpox – not by exterminating the ill, but by a general vaccination that eliminated the virus’ hiding places. This method isn’t working so far to eradicate the rampaging measles, because of the government’s slow and weak response, people’s refusal to get vaccinated and thereby protect themselves and others, and infected individuals freely moving in the population.

The model that the Canadians developed allows prediction of the behavior of real diseases that have an incubation period. In 2013, a pair of American researchers improved the model so that it better represents zombies as they are portrayed in the older movies, and on the way, can also predict with some accuracy the spread of flu and other real diseases.


A cockroach defending itself by kicking a parasitic wasp. Photography: Kenneth Catania, Vanderbilt University.
To hit the zombie in the head as hard as possible. A cockroach defending itself by kicking a parasitic wasp. Photography: Kenneth Catania, Vanderbilt University.

Where were you when the apocalypse broke out?

In 2015, another group of researchers tested what would happen if a zombie outbreak would take place in the United States. As expected, the group’s model showed that the most dangerous places are urban centers and other crowded spaces, where it’s easy for the zombies to attack and infect new victims. Sparsely populated areas, such as the Rockies, are the safest.

After 28 days, the danger in urban centers decreases, and the open spaces amid cities become more dangerous for humans. But obviously, if humans escape back into the city centers, the zombies again would have who to infect there and the danger increases once more.

The researchers also used epidemiological models to predict the spread of the disease, applying them to the features of modern zombies – there is no reversing the zombie condition, and it is not fatal – a zombie would only die if someone kills it.

The model integrated data on the odds of getting infected against the odds of killing a zombie, which were calculated based on watching Shaun of the Dead and other zombie films; this is how the researchers determined the rate of the disease’s spread and changes in it over time. These models and assumptions are available through a zombie plague simulator created by the researchers; you are welcome to try to destroy humanity in a zombie apocalypse.

Nevertheless, if you find yourself in a crowded city, it is probably best to escape to the nearest mall, where there is food and lots of hiding places. According to a paper published in 2009, which wasn’t directly related to zombies, the odds of successfully escaping from predators that “move randomly” – like some of the zombies, before they see their prey – are greater when the space you hide is more complex, with many twisting routes. If the zombies somehow do find you, take a tip from the cockroach that is defending itself from the wasp trying to turn it into an undead: Protect your body and hit the zombie in the head violently, again and again, with an axe, nail-studded club, or a barbed-wire wrapped baseball bat named “Lucile”.

Until the apocalypse arrives, don’t be a zombie: Prepare for emergencies and protect yourself and the public from disease-causing agents. Get vaccinated according to the Ministry of Health’s recommendations, maintain good hygiene, sneeze into your elbow and not into your hand, and if you have an infectious disease, don’t spread it. And most importantly, stay at home and try to fend off the impulse to bite people.