The spore is ten times thinner than a human hair, but that is enough to make it a lethal threat. It only needs an ant to walk over it. The insect has no way of knowing, but at that very moment the tiny cell has attached itself to its exoskeleton, penetrated it, and now a parasite is developing inside, growing until it reaches the nervous system. No one can explain how it does this, but in a very short time the fungus takes control of the ant and bends it to its will.
Days later, in a completely unusual move, the little worker ant leaves the path that connects its anthill with the rest of the jungle. It climbs up a tree trunk until it finds a leaf — not too high, not too low — and bites into it with its mandibles. The ant dies, and its executioner finally reveals itself: from the insect’s head sprouts the imposing stalk of an Ophiocordyceps unilateralis, which now releases spores that will patiently disperse until they find new victims.
This infectious fungus, specialized in attacking ants, altering their behavior, and using them to reproduce, exists in the real world but became famous through fiction. In the video game and TV series The Last of Us, there are fungi that parasitize and zombify humans. These are the Cordyceps, relatives of the ones that target ants. But it’s not the only parasite capable of controlling its host. In fact, there is a name for the victims: zombie insects.
American science journalist Mindy Weisberger has just published Rise of the Zombie Bugs, a book that explores the unsettling phenomenon of zombification in nature, far from the Hollywood spectacle.
“There’s something about zombies that I find particularly intriguing,” Weisberger says in a video interview with EL PAÍS from New York, where he lives and collaborates with museums and science documentaries. “The idea of losing free will, of something external controlling your body while you’re still technically alive, is unsettling,” he adds. Parasites reproduce by rewriting the neurochemistry of their victims, transforming them into “the living dead.” Viruses, worms, fungi, and wasps: the list of zombie-like species in the real world is long and varied.
Although these unsettling relationships between species have existed for millions of years, entomology still doesn’t fully understand how they work. “Scientists are just beginning to unravel the details. How they manipulate, what chemical pathways they use, what neural mechanisms are involved — it’s all very mysterious and fascinating,” explains Weisberger. But to understand zombification, we must first understand parasitism.
The term “parasite” was first used in the 16th century, and its origins can be traced back to ancient Greek, where parasitos means “person who eats at the table of another.” “It’s different from a symbiotic relationship,” Weisberger writes, “because in those cases, both living beings enjoy the benefits. When it comes to a parasite, it’s the only one who benefits from the arrangement.”
The most cinematic example — and the favorite of the science communicator — is that of the so-called “zombie snail.” The worm Leucochloridium paradoxum begins its life in bird droppings, where its eggs are accidentally ingested by a snail.
Once inside, the larvae hatch and travel to the mollusk’s tentacles, swelling and shaking them to resemble a caterpillar. The parasite takes control and forces the snail to leave the shadows and expose itself in broad daylight. Birds, attracted by the caterpillar-like appearance, peck at it, allowing the worms to enter the bird’s digestive system. And so, the cycle begins again. “It’s a complex cycle, but visually impressive and evolutionarily fascinating,” notes Weisberger.
Millions of years of parasitic relationships
The first evidence of a parasitic relationship comes from the sea and dates back 500 million years. These are the remains of small invertebrates called brachiopods that inhabited an ocean that occupied present-day southern China. Preserved parts of their shells reveal mineralized tubes constructed by tiny worms that likely stole food from their hosts.
“The parasitologists I interviewed for the book joke that the first life form was free, and the second was parasitic,” says the author. Of the approximately 7.7 million known animal species, an estimated 40% are parasitic. And the strategy has evolved independently at least 223 times throughout history.
When asked how these various controlling strategies developed, Weisberger responds that for researchers, “it’s difficult to know because parasitic behavior is complex, and many relationships cannot be replicated in a laboratory.” But there are clues.
It is known, for example, that many zombifying parasites do not introduce new substances into their victims but rather manipulate the chemistry already present in their hosts and use it to their advantage. In other cases, it is almost as if they “drug” them, as happens with jewel wasps. These insects turn cockroaches into functional zombies that serve as living shelters for their larvae.
The process is surgical: the wasp first stings the cockroach in the thorax, paralyzing its front legs. Then, it delivers a second sting directly into the brain, where it takes control of decision-making and the escape instinct, causing the cockroach to obey and ultimately become fresh food for the wasp larvae, which eat it alive. “Although this is exceptional, and in most cases there is no chemical silver bullet explaining the behavioral change,” the author notes.
The case of mammals
The hit TV adaptation of The Last of Us, in which a fictional species of fungi triggers an apocalypse, reopened a debate that occasionally resurfaces in some corners of the internet. Could a parasite zombify a person? “No, I don’t think we have to worry about a fungal zombie pandemic,” Weisberger says.
Fungi don’t thrive inside bodies with high temperatures like those of mammals. “In fact, it’s thought that one of the reasons we evolved with such a high body temperature was precisely to protect ourselves against fungal infections,” the author explains.
For now, insects are the only ones who should worry about these fungi. Oscar Soriano, researcher at the Department of Biodiversity and Evolutionary Biology at the National Museum of Natural Sciences in Madrid, agrees. “I find it more complicated for one of these parasites to manage to control more complex structures, like the brain of a mammal,” he says. Although he adds a caveat: “Still, look at the effect drugs have. Some manipulate the human brain by producing hallucinations and making it act accordingly. Maybe it’s just a matter of the right molecule appearing.”
The relationships between parasites and zombie insects are highly specialized. They are very precise mechanisms that have taken millions of years of evolutionary trial and error. “To think that such a unique parasite could suddenly jump in and take over a human brain doesn’t make much sense from an evolutionary perspective. It would be like trying to use one key for a completely different lock,” says Weisberger.
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