In the tropical rainforests of South America, Asia and Australia, there thrives a parasitic fungus (Cordyceps) that creates real life zombies.
This astonishing disease prevents the host from controlling its own behaviors - making actionable choices on its behalf and overriding innate survival instincts.
We're not talking human zombies - don't expect Word War Z just yet - but rather insect ones.
The most extreme zombification takes place in ants, in three bizarre phases:
To create a zombie-ant, first a Cordyceps fungal spore must attach itself and use enzymes and pressure to blow a hole into the ant's body.
"We know from studies of fungal parasites of plants, particularly rice, they can build up a pressure inside their spore equivalent to the pressure in the wheel of a 747," says David Hughes, a behavioral ecologist at Pennsylvania State University, who studies the disease.
With its genetic material blasted into the ant host, the fungi grows rapidly. It then releases a cocktail of mind-controlling chemicals into the ant's brain and begins to take over its conscious behaviors.
Within two weeks of the invasion, the ant leaves his colony and seeks a specific location in the forest, guided by the parasitic fungi.
He heads for a target destination preferred by the fungus - the perfect warm, humid environment from which Cordyceps can perform its next horrifying act.
With remarkable precision, Hughes explaines, the ant is "manipulated to bite onto a very specific location on the underside of a leaf, the main vein of a leaf, leaves orientated north, northwest, roughly 25cm off the ground."
According to Hughes, it's one of the most complex examples of parasitic manipulation. To the rest of us, it's like zombification.
And the weirdest bit is yet to come...
All of this happens around solar noon, which underpins the shocking level of precision orchestrated by the fungi.
Then the zombification takes another bizarre twist.
At dusk, the ant dies completely, and the fungi enjoys a long cool night to sprout a spore-producing structure -- which grows out of the ant's head.
When the sprouting is complete, fungal spores are released into the air and new ants are infected, further spreading the fatal disease to other ants.
Take a look at these astonishing images of fungal stalks emerging from the bodies of insects. (Ants are not the only victims: there are suspected to be thousands of the Cordyceps fungi, each tailored to different species of insects.)
Science-fiction has not overlooked nature's own take on the zombie genre.
The Last of Us, released in 2013 on PlayStation 3, is based on the notion of the Cordyceps fungi transferring among humans.
Set 20 years after a fungal parasite has wiped out most of humanity, the game involves survival in a gruesome zombified world.
Though clearly a work of fiction, the game is inspired by the real life zombie ants produced by Cordyceps. It tempts the question: could an infectious, zombie-like disease transfer to humans and affect us in equally deadly ways?
The answer lies within a couple of factors:
Hughes, who was a consultant on The Last of Us, points out that 60% of all human diseases have jumped from animals.
"AIDS is a classic example," he says. "It jumped from being in a chimpanzee - where it doesn't seem to be particularly virulent or dangerous - into humans. Consequently 75 million people have become infected by HIV; 35 million people have died."
Hughes also points to Avian flu as having transferred from animals to humans.
"The classical example is the 1918 Bird Flu. After the First World War, it killed more individuals [an estimated 50,000,000 to 100,000,000] than the First World War itself. That jumped from an avian source into humans."
So how about a parasitic fungus?
We inhale billions of fungal spores every day - and our immune system has evolved to deal with that threat. However in the right conditions, fungal diseases do breach the immunity barrier.
We also know that fungal diseases can jump from animals to humans. These are called zoonoses. They can also occur in bacterial, viral or parasitic form. Numerous zoonotic diseases kill 2,200,000 people every year:
So it is possible that an existing fungal disease from the animal kingdom could evolve to infect and harm humans. Indeed, three-quarters of emerging infectious diseases now come from animals.
Once active in the human population, a fungus would need to spread itself among people in order to be really successful.
The key is to spread fast - before we can identify a treatment. And on that front, we're certainly offering a helping hand, providing immediate distribution channels with global air travel.
What's more, we're turning the temperature up on the planet. Fungi love warmth - and we're creating more environments in which they can survive.
Our rapidly-growing population and cramped city living create superb environments for disease to spread quickly. With fungal spores spreading airborne, we wouldn't even need direct human contact or mingling of blood (as in most zombie movies) for infection.
If the disease mimicked the ant-targeted Cordyceps genus described here, it would be important for us to quarantine infected individuals before they become contagious, and to dispose of corpses before the spore-producing stalks emerge.
Of course, there's something we're overlooking here. It's the fact that the human brain differs significantly from the ant brain.
Are ants even conscious, as we understand it? We don't consider them to be animals with self awareness. But they do demonstrate many social and sentient behaviors.
Even so, could a fungus develop the complexity to override our conscious free will?
Humans are certainly not like ants; we are arguably more complicated. And despite advances in our knowledge of psychiatry and biology, we are still overwhelmed by the myriad factors that influence our behavior (think: genetics, upbringing, trauma, diet, hormones, and so on).
Is it even possible that a human-specific fungal infection could so completely override our conscious choices and behaviors -- the way that Cordyceps overrides an ant's innate survival drive to stay with the colony?
Well, perhaps. We may not fully understand the root of our behaviors but fungi like Cordyceps have been evolving for almost 50,000,000 years. They seem to know exactly what they're doing when they manipulate their host's behavior.
For example, ants infected by roundworms flaunt themselves in such a way that makes them more likely to be eaten by birds, thus spreading the worms further. A spider infected by a wasp larvae will spin a different pattern in its web. Crickets are driven to drown themselves upon infection by hairworms. And rabies is famous for ramping up aggression and salivation before jaw paralysis.
All of these changes, designed to ensure the continued survival of the parasite, involve direct manipulation of the victim's behavior.
Is it possible, then, that a parasite has already evolved to influence human behaviors? Just how likely is this prospect?
Michael Eisen, a professor of molecular and cell biology at UC Berkeley, suggests there is a lot of complexity we don't understand: "We see examples of parasite manipulation across all taxa - insects, shellfish, even llamas can be infected. But what's still lacking is an understanding of how microbes can accomplish some pretty sophisticated things. Some of these parasites are only one single cell."
What Eisen finds most fascinating is how perfectly tuned each microbe's response is to its host. He's now exploring this manipulation, including a study on the role of Toxoplasma gondii, a parasite renowned for its putative role in human behavior.
Only recently, scientists discovered that in mice,Toxo has several genes which affect dopamine production (responsible for signaling pleasure, motivation and fear). It's natural to wonder whether Toxo - to which half the human population has already been exposed in some countries - might affect our fear, pleasure, anxiety and arousal through the manipulation of our dopamine levels.
In 2011, Professor John Cyran at University College Cork in Ireland found that mice that ate Lactobacillus (a common bacteria found in yoghurt) had lower levels of stress hormones in stressful situations. They also had higher levels of GABA, a neurotransmitter involved in anxiety and stress responses.
How could bacteria in the gut create immediate changes in brain chemistry? The researchers fancied the vagus nerve as a conduit - carrying information from the intestine to the brain. When they severed the nerve in mice, the effect ceased.
At this stage, we'd be getting ahead of ourselves to suggest that Toxo or any other parasite causes changes in human behavior.
But the evidence in other animals is present - and mounting.
In nature there are plenty of examples of microbes causing behavior modification in their hosts. Some even cause zombification or death of the host.
We haven't seen anything like this occur in humans and there is no data to support the emergence of any zombie-like disease with such drastic influences.
However further research might reveal some surprising truths. We live at the mercy of our environment - evolving over millions of years alongside fungi, bacteria, viruses and parasites which evolve inside us.
We are a collection of cells -- and signaling pathways are often shared between living things. It's a fact that molecules released by bacteria interact with our cells. As Wendy Ingram, a graduate from Eisen's group, puts it: "We're not complete puppets, but we do have some strings that microbes can use to pull us in different directions."
You may argue that the relative simplicity of an ant's conscious will may be more easily suppressed by an invading parasite than a human mind.
But there is nothing to say that a parasite, tailored over time to chemically manipulate our minds, could never evolve. Just as the human mind itself evolved. And just as Cordyceps evolved to exploit and create the zombie-ant.