Imma Perfetto
Cosmos science journalist
From parasitic wasps to body-hijacking fungi, the natural world is full of real-life zombification. But could our favourite post-apocalyptic TV shows come true?
Imma Perfetto investigates. Listen on, if you dare.
The Last of Us, The Walking Dead, World War Z, Zombieland, Shaun of the Dead… I could keep going but I think you get the gist.
Zombies are everywhere in fiction, from movies to TV shows, video games to books.
These stories are usually pretty hand-wavey about the actual science that has turned humans into mindless, shambling, killing machines, but you might be surprised to know that there are actually some pretty gruesome examples of zombification in real life.
If you can stomach it, come with me on a tour into the lives of parasitic wasps, worms, single-celled organisms, fungi, viruses – and even into our own brains.
Horror authors and script writers, listen closely – because we all know that some of the scariest stories are the ones rooted in reality.
I could keep going – there are so many more thrillingly grisly examples of zombification in nature – but you might need some fresh air and recovery time.
“Nature is rife with examples of parasitic puppeteers infecting their host.”
Just one more thing before you go. These examples of zombification fit the major zombie stereotypes: increased aggression, a loss of autonomy and a compulsion to bite or to ensure the spread of the parasite or virus infecting the host. But thankfully, there aren’t any known diseases or afflictions in nature that can continuously reanimate corpses – so the undead remain firmly within the realms of fantasy.
Maybe that will help you rest easy at night – or maybe I’ve already infected your nightmares with mind-controlling worms and fungi that will consume you from the inside out.
Sweet dreams.
Not your typical swimming lesson
The first stop on our super fun and not-at-all distressing zombie extravaganza is parasites. These are organisms that live on or inside an organism of another species, its host.
Gordian worms, also known as horsehair worms, are long, thin parasitic worms found in fresh water all over the world. The larvae bore into the body of a host, initially other larvae in the water, which then get snatched up and eaten by unfortunate insects like grasshoppers or crickets. There, the worm grows inside the insect’s body cavity to as long as 30 centimetres, secreting digestive enzymes from their skin to absorb their host body’s nutrients.
If that isn’t spine-chilling enough, it gets worse. Hosts infected with a gordian worm will, despite avoiding water in all other cases, perform a deadly cannonball into the next aquatic environment they come across and drown. The worm then bursts out of a borehole and swims off in search of a mate, starting the cycle all over again.
How the worms manage to manipulate the insects’ cognitive functions is not fully understood, but researchers believe the worms produce molecules that act on the development of their host’s central nervous system, thus altering physiological responses and behaviours.
Home sweet zombie spider home
If those tapeworms from hell made you squeamish, get a sick bucket ready because there are far worse parasites to contend with.
Nature is rife with examples of parasitic puppeteers infecting their host and inducing some pretty strange behaviour. One of my favourites is from a 2018 study published in Ecological Entomology, which discovered a previously unknown species of wasp deep in the Ecuadorian Amazon jungle – with the ability to transform a species of spider into a zombie-like drone.
These spiders (Anelosimus eximius) are known for living together in large colonies, where they cooperate to hunt and parent. Think Aragog and his giant Acromantula children chilling together in Hogwarts’ Forbidden Forest.
But the study, led by scientists from the University of British Columbia in Canada, found that Zatypota wasps intrude on this idyllic social life by laying eggs on the abdomens of the spiders, which then hatch into larvae that feed on the spider’s internal body fluid. The larvae finally take complete control of the spider’s body, hijacking its brain and triggering some unusual behaviour.
The researchers don’t know yet how the wasps manage total control, but suspect it may involve the wasp larva injecting hormones into the spider.
The result is that the spider does something against its own behavioural tendencies: it leaves the colony and builds a densely woven, cocoon-like nest where the larvae can grow safely and comfortably into adult wasps, devouring their host in the process.
Delicious.
This fungus is trending
We can’t cover real-life inspirations for zombie apocalypses without talking about Cordyceps. It’s possibly the world’s most infamous fungus thanks to its starring role in the video games The Last of Us Parts I and II, as well as the live-action TV adaptation released earlier this year.
Cordyceps and Ophiocordyceps are genera of fungi that include about 750 species worldwide, most of which are parasitic and infect insects and other arthropods, like carpenter ants and trapdoor spiders. Infection starts when a fungal spore gets inside an organism, like an ant, and begins growing.
The ant’s behaviour gets hijacked and it prioritises its parasite’s reproduction over everything else. It stops foraging for the colony and communicating with its nest mates, becomes hyperactive and wanders off on its own to find a spot to climb up to.
It then chomps down on a piece of vegetation in a move called the “death grip”. The muscles in its mandibles then atrophy, locking it in place for the final throes of its life.
The fungus consumes everything inside the host, killing it, then uses those nutrients to sprout a fruiting body out the top of the host’s head. Spores form and drift off to infect more unwary hosts.
How is this piece of horrid biology possible? Again, it comes down to secreted chemicals and their effect on the host’s physiology. For example, a 2015 study led by researchers at Pennsylvania State University in the US identified a range of secreted proteins produced increasingly by the fungi during the strange biting behaviour. These may affect a range of processes including immune responses, stress responses and impairing the production of chemicals used in communication between insects.
It’s a viral sensation
This deep-dive into grossity wouldn’t be complete without mentioning rabies – the virus that inspired the zombies of the horror classic 28 Days Later.
A rabies infection has all the symptoms of your typical zombie: a compulsive need to bite, a fear of light and mindless aggression. The virus is usually transmitted through bites and scratches from an infected animal, and it takes time to travel to the brain before causing symptoms – which is absolutely essential if you’re going to have the requisite scene where a person tries to hide their bite, but gets progressively sicker until the group confronts them and finds out they’re infected.
This incubation period typically lasts 2–3 months for rabies, but can vary from one week to a year. Rabies progresses to the central nervous system where it causes inflammation of the brain and spinal cord, triggering symptoms in two different forms.
There’s paralytic rabies, which occurs in about 20% of human cases. Here, muscles become progressively paralysed, and the person falls into a coma before dying.
Then there’s furious rabies, like your more World War Z flavour of zombie, which causes irrational aggression, hyperactivity, hallucinations and a fear of water and fresh air.
A catastrophe waiting to happen
A little closer to home, there’s the single-celled parasite Toxoplasma gondii that infects the brains of our kitty cats, and ours too.
A 2014 study published in PLoS ONE actually estimates that up to half of the world’s human population is infected, though most of us have no symptoms. That’s pretty alarming to contemplate, since Toxoplasma has been shown to change the behaviour of infected mice, who become hyperactive and lose their innate fear of cats – in some cases even appearing to be attracted to them.
There’s evidence that also suggests toxoplasmosis might be linked to personality changes in humans too. One study found an association with impulsivity in younger men and increased aggression in women, while another found a positive association between national homicide rates and prevalence of the parasite in the population – although correlation is not causation.