You’re in a dark forest, surrounded by misty fog and strange animal noises. You’re on a mission, jumping over traps and snares in search of something you’ve lost dear to you. As you make your way deeper into the forest, you come across an odd glowing plant hanging from a nearby tree branch, perhaps a fungus of some kind. As you creep up to inspect the glowing substance, you are horrified to have it drop onto your head and cling to your scalp. You try to scream, but your eyes go dark. You find yourself walking, but you don’t know where to, your arms heavy and your feet numb. Is it possible you no longer have control of your body?

This is a scene from Limbo, a modern video game that is a great example of a post-apocalyptic genre filled with zombie-themed obstacles. Zombies have long been a part of our storytelling culture, with stories about zombies originating in the Afro-Caribbean spiritual belief system of Voodoo and the belief that witchcraft could raise corpses from the dead. But modern scientific discoveries of mind-controlling parasites may be further inspiring media examples of this favorite American obsession.

"Zombies seem to be one of the most popular metaphors these days,” said Tracy Stephenson Shaffer, an associate professor in the Department of Communication Studies at Louisiana State University whose research interests include popular culture and the horror genre. Shaffer is also something of a zombie expert; in 2009 she produced Nonfiction Zombie, an ensemble performance considering the zombie's recent rise in popularity as a useful metaphor and critique of contemporary life.

“They are often used to describe anything that has lost control or acts in a mindless way,” Shaffer said. “They've captured the public's imagination because they are both us and not us simultaneously. They are also dead – our biggest fear – which makes them a great monster."

Zombies have morphed over the years, however, embodying, or rather em-monsterfying, different metaphors in popular culture. The “disease model” of zombies is apparently relatively recent in our entertainment culture. Today’s zombies are often called “the infected,” and zombie outbreaks spread like other infectious diseases. Mathematicians have even created models of zombie infection outbreaks and models of how to fight zombies off.

The background stories behind zombie movies, video games etc, are purposefully vague and inconsistent in explaining how the zombies came about in the first place. Some ideas include radiation (Night of the Living Dead), exposure to airborne viruses (Resident Evil), mutated diseases carried by various vectors (Dead Rising claimed it was from bee stings of genetically altered bees). – When Zombies Attack! Mathematical Modelling

But where does our continuing imagination for zombie infections come from? Could traditional zombie fiction be morphing into a more scientifically inspired genre?

Zombie Ants

David Hughes, Penn State assistant professor of entomology and biology, is an expert on ‘zombie’ ants. Through research in Thailand and Brazil, Hughes and other scientists have revealed the biology and mechanics of Ophiocordyceps unilateralis, a parasitoidal fungus that infects particular ant species, dramatically changing behavior in order to benefit fungal reproduction.

“The fungi which control ant behavior are an interesting example of adaptation through natural selection,” Hughes said during a Penn State ScienceCast video. “The life cycle of the fungus begins with a fungus spore on the forest floor. And when the ants are out foraging for food, they pass through these little ‘killing fields’, as we describe them, of fungal spores.”

I don’t know about you, but I’m already imagining “killing fields” of zombie pods in my mind, a combination of Invasion of the Body Snatchers and The Walking Dead.

The growing Ophiocordyceps unilateralis fungus fills the ant’s body and head, affecting the central nervous system of the ant and causing confusion and convulsions. The fungus eventually guides infected ants to the underside of hanging leaves, forcing each infected ant to bite the main vein of a leaf, and, through a lock-jaw phenomenon, keeping the ant in this location until a fungal fruiting body grows through the ant’s head and releases more fungal spores for another deadly life cycle.

When I asked Hughes in an email exchange what he thought the coolest part of this deadly ant-fungus relationship was, he said, “[t]hat complex manipulation of animal behavior is controlled by a single-celled microbe."

When complex animal behavior can be controlled by a single microbe, a zombie human apocalypse starts to sound not-so-far-fetched. The pure science isn’t the only interesting aspect of the discovery of a real-life zombie ant fungus. In his fascination with zombie behavior, “where a parasite actually takes over the brain of its host and causes the host to do its bidding,” Hughes has himself guided the integration of his science with Hollywood movie and gaming culture.

“…I'm motivated by this opportunity to get the science to people who wouldn't get it in another way,” Hughes said in a Penn State news article.

Just the fact that the ant struck by Ophiocordyceps unilateralis has been commonly termed the “zombie ant” places this biological phenomenon not only in a scientific context, but also in a cultural context. As a part of a society obsessed with monsters, it isn’t odd that Hughes and his colleagues refer to Ophiocordyceps unilateralis as the zombie ant fungus.

"The 'zombie ant' is interesting because it has more in common with the voodoo zombies of early film,” Shaffer said. “The first zombie film, the Halperin brothers 'White Zombie' (1932) stars none other than Bela Lugosi as a voodoo master who controls the minds of zombies to do his bidding. The fungus seems to work like a voodoo master over the ants."

Aha. So we might not be as obsessed with the infected zombie ant as we are with the voodoo master fungus Ophiocordyceps unilateralis that makes ants do its bidding. While we may have no control over zombies, we might be inspired to harness the powers of the fungus “master” to do our bidding. Hughes, for example, hopes to perhaps someday use Ophiocordyceps fungi as a biological insecticide to target pest insects.

"It will be a major focus in my group," Hughes said in a 2011 National Geographic News article. "How can we use this [discovery] to control ants, which are, after all, devastating pests in many places?"

We can start to see how both science and the zombie metaphor are put to work in popular culture.

All Zombies Evolve

Fungi aren’t the only zombie voodoo masters. Parasite-induced alteration of host behavior has been also been reported in viruses, bacteria, protozoans, nematodes and other microscopic creatures. The changes that “zombie” parasites induce in their hosts range from small shifts in the percentage of time the host spends performing certain activities to the display of spectacularly abnormal behaviors, like in the zombie ant. Changes in the host caused by infection by the “zombie” are beneficial to the parasite because they lead to better transmission, or reproduction, success of the parasite.

Zombie parasites that dramatically change host behavior, like the zombie ant fungus, also likely evolved from ancestor parasites that either did not change behavior, or only changed a single behavioral trait. Over generations, however, natural selection would favor those parasites that could control more than one dimension of host behavior. For example, the parasite Polymorphus minutus, which starts out its life cycle in crustaceans and ends it in birds that eat the crustacean, such as the Mallard Duck, causes the crustaceans to swim to the surface of the water when a bird is stirring around in the water, for example. However, the parasite has also evolved to increase the swimming speed of the crustacean so that it can avoid water predators that are not hosts for the parasite.

As it turns out, zombie parasites have evolved over the years, just as zombies as a cultural phenomenon have evolved to embody different metaphors in our lives.

Another parasitic protozoan, Toxoplasma gondii, infects rodents, cats and even humans. The parasite can only reproduce in the intestines of felines, so cats are the preferred host. This zombie parasite manipulates the behavior of rats, making them rather reckless and attracted to the smell of cat urine (gross!), thus increasing the rodents’ chances of being preyed upon by cats. Some studies have even suggested that T. gondii may slightly change behavior of infected humans, potentially playing a role in schizophrenic disorders. A real-life zombie voodoo master of humans?! Not so fast – research is still needed to confirm this potential association between T. gondii and behavioral changes in humans, and to determine whether this is true host manipulation, or, more likely, just a side effect of infection.

From Culture to Science and Back Again

Fascination with zombies may have started with ancient spiritual beliefs in voodoo and magic, but the scientific community hasn’t been immune to the bug. In the process of calling insects and animals infected by behavior-changing parasites “zombies,” scientists themselves are evoking metaphors that pervade our culture far beyond the science itself.

‘”There is something horrifying and wondrous about a tiny ‘implant’ being able to control such a large animal machine,” Journal of Experimental Biology Editor Michael Dickinson was quoted in a recent scholarly article titled How Pernicious Parasites Turn Victims into Zombies. “‘Neuroparasitology is a science where science meets science fiction.”

It appears that zombies as a cultural metaphor are not only helping us deal with scary subjects such as infectious disease epidemics and global changes beyond our immediate control, but also helping us put complex biological interactions between different organisms in a context we can relate to.

And the zombie bus doesn’t stop there. Hughes is the science advisor to the movie World War Z, featuring Brad Pitt and a zombie epidemic, and Last of Us, an extremely popular video game featuring his own Ophiocordyceps unilateralis making the jump from insects to humans.

"In terms of video games, I might flinch if the game asked me to kill other humans, but a zombie?” Shaffer said. “Killing a zombie is a bit easier."

And so, just like a zombie parasite life cycle, the zombie metaphor comes back full circle to popular culture. As much as it is a spectacular vehicle for communication of complex science to the public, it may be first and foremost a cultural phenomenon created out of our deepest human fears and concerns.

Learn more about Zombie Ants:

Getting to the bottom of the zombie ant phenomenon

Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection