Zombies have become staple figures of popular culture, and the zombie apocalypse is a trope that features in many books, movies, and TV series. But are there actual, real cases of zombiism in nature? Read this special feature to find out.

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Are there any real cases of zombification? We investigate.

Zombie. The walking dead. Reanimated corpses. The undead.

Whatever you choose to call them, these corpses that rise from the grave to walk the world and terrify — and sometimes infect — its inhabitants are one of the top monsters in popular culture.

The word zombie — originally spelled as zombi — first came into the English language in the 1800s, when poet Robert Southey mentioned it in his History of Brazil.

According to the Merriam-Webster dictionary, the word comes from the Louisiana Creole or Haitian Creole word zonbi, and it is akin to the Kimbundu term nzúmbe, which means ghost.

The word refers to creatures from Haitian folklore that, at its origin, was little more than the ghosts from Western folklore.

However, little by little, the concept evolved to refer to a person that is rendered mindless by a witch doctor, entering a death-like state while still animated, and thus becoming the witch doctor's slave.

Nowadays, people use the word "zombie" a lot more loosely — often metaphorically — to refer to anyone or anything that presents as apathetic, moves slowly, and demonstrates little awareness of their surroundings.

But do zombies, or zombie-like beings actually exist in nature, and if so, what are they, and how do they come to enter this state of "undeath?" And can humans ever become zombie-like? In this special feature, we investigate.

1. Zombie ants

Ophiocordyceps is a genus of fungi that has more than 200 species, and mycologists are still counting. Many species of fungi can be dangerous, often because they are toxic to animals, but there is one thing in particular that makes Ophiocordyceps especially frightening.

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Carpenter ants taken over by parasitic fungi give in to their attackers and 'lose their minds.'

These species of fungus "target" and infect various insects through their spores. After infection takes place, the parasitic fungus takes control of the insect's mind, altering its behavior to make the propagation of fungal spores more likely.

Ophiocordyceps "feed" on the insects they attach to, growing into and out of their bodies until the insects die.

One of these species, Ophiocordyceps unilateralis sensu lato, specifically infects, controls, and kills carpenter ants (Camponotus castaneus), native to North America.

When Ophiocordyceps unilateralis infect carpenter ants, they turn them into zombies. The ants become compelled to climb to the top of elevated vegetation, where they remain affixed and die. The high elevation allows the fungus to grow and later spread its spores widely.

Researchers from Pennsylvania State (Penn State) University found that O. unilateralis take full control of the ants' muscle fibers, forcing them to move as it "wants" them to.

"We found that a high percentage of the cells in a host were fungal cells," notes David Hughes, who is associate professor of entomology and biology at Penn State.

"In essence, these manipulated animals were a fungus in ants' clothing."

David Hughes

Below, you can watch a video showing how the parasitic fungus infects its victims, leading them to their death.

In 1997, the two published a study paper in The Lancet in which they analyzed the cases of three individuals from Haiti whose communities had identified as zombies.

One was a 30-year-old woman who had, allegedly, quickly died after having fallen ill. Her family recognized her walking about as a "zombie" 3 years after this event. Another was a young man who had "died" at 18, and reemerged after another 18 years at a cockfight.

The final case study concerned another woman who had "died" at 18 but was spotted again as a zombie 13 years after this event.

Dr. Douyon and Prof. Littlewood examined the three "zombies," and found that they had not been the victims of an evil spell. Instead, medical reasons could explain their zombification.

The first "zombie" had catatonic schizophrenia, a rare condition that makes the person act as though they are walking in a stupor. The second person had experienced brain damage, and also had epilepsy, while the third appeared merely to have a learning disability.

"People with a chronic schizophrenic illness, brain damage, or learning disability are not uncommonly met with wandering in Haiti, and they would be particularly likely to be identified as lacking volition and memory which are characteristics of a zombi," the researchers write in their paper.

But there is also a specific psychiatric disorder called Cotard's syndrome that can cause people to act like zombies. This is because they are under the delusion that they are dead or decomposing.

It remains unclear just how prevalent this condition is, but research suggests that it is a rare occurrence. Documented cases of people with Cotard's syndrome are unsettling, nevertheless.

One case study reports the situation of a 53-year-old woman who "was complaining that she was dead, smelled like rotting flesh, and wanted to be taken to a morgue so that she could be with dead people."

Another speaks of a 65-year-old man who had developed a belief that his organs — including his brain — had stopped working, and that even the house in which he lived was slowly but steadily falling apart.

At some point, the man attempted to take his own life. Researchers report that "[h]is suicide note revealed that he wanted to kill himself as he feared spreading a deadly infection to the villagers who resultantly might suffer from cancer."

Do such cases mean that zombies are real in some way, or, just as our fascination with the figure of the zombie in folklore and popular culture, do they merely reflect our uneasy relationship with death? We leave it to you to decide.