The rise of the zombie bugs

Above: Fly infected with Ophiocordyceps fungus by Nicky Bay

26 May 2025

Mindy Weisberger on the unsettling science of parasitic mind-control

Recently the internet seems to be awash with horrifying clips of ‘zombie’ spiders and ants forced into strange behaviours by a fungus or virus that will soon kill them – often rather gruesomely.

However, as well as being morbidly fascinating, ‘zombification’ is a topic of interest for a growing number of entomologists, microbiologists and neuroscientists for what it can tell us about host-parasite relationships and animal behaviour. Indeed, zombifying parasites have been dubbed ‘nature’s neuroscientists’ for their ability to rewire their hosts’ nervous systems to incredible effect. 

Science journalist Mindy Weisberger, author of the new book Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control, tells The Biologist all about the weird and wonderful world of zombifying parasites, and why we humans seem drawn to stories about zombies time and again. 

How did you become interested in this bizarre form of parasitism?

Before I was a science writer I was a filmmaker, and one thing that filmmakers love is films that can get made quickly and for very low budget. Zombie movies have always been part of that and you may have noticed they seem to be having a bit of a moment once again. 

The first time I became aware zombification existed in the natural world was while researching a video for the American Museum of Natural History. The idea that this level of behaviour manipulation was possible was just fascinating. The deeper I delved, the more I saw how successful it is as a strategy. It’s actually fairly widespread – especially in parasites of arthropods. By studying how behaviour can be manipulated in this way, there’s a lot to be learned about how brains are wired.

There seem to be lots of articles and video clips of zombified insects around now, but I’d never heard of it until a few years ago. How far back do reports of parasitic mind control go?

A lot of the information about how zombification works has really come together in the last few decades, but in terms of observations there are records going back to the 19th century, when scientists may not necessarily have known what they were seeing.

For example, the naturalist Benjamin Banneker wrote one of the first observations of periodical cicadas infected by a zombifying fungus. This particular fungus, Massospora cicadina, multiplies in the cicada’s body and weakens its exoskeleton to the point where the last third of the abdomen just sloughs off. Banneker observed that the infected cicadas were, bizarrely, still merrily producing their mating call even as their bodies were rotting away. We now know this fungus releases chemicals that hypersexualise the cicadas, so even though their bodies are falling apart, they’re still driven to mate, spreading the fungal spores.

A greater understanding of how zombification works has developed over the last few years.
Tim Fogg / Gilles San Martin

There are also examples from 19th century literature of flies behaving strangely, and over centuries mycologists and entomologists have been able to piece together why the insect looks and behaves the way it does. Fast forward to now and you have scientists doing brilliant genetic and molecular work to unravel exactly what is going on and how the zombifier is making its host change its behaviour.

It’s easier nowadays to create rich networks of communication between scientists in different disciplines, which is pushing things forward in this area.

Is there a scientific definition of ‘zombification’?

There are many ways pathogens can change their hosts’ behaviour. However, in the case of zombification there is a clear behaviour change that only benefits the zombifying organism.

Take horsehair worms, for example, which need to reproduce in water. Their host is typically a terrestrial insect and in the late stage of infection they manipulate that host to jump into water. This unusual behaviour does not benefit the host in any way. 

Can you give us a sense of how zombifiers can manipulate behaviour like that? The pathways from neurochemistry to a specific action or behaviour like jumping into water seem so hard to unpack and understand, let alone manipulate precisely.

Yes, these parasites understand the neurochemistry of their host better than scientists who have spent their lifetimes studying them. The general understanding is that there can be different pathways that are part of the zombifying package. There is the physical – for example, the presence of mycelia growing through the nervous system and the consumption of tissue, which can change host behaviour. Then there’s the chemical – the zombifier may release mind-altering chemicals, or stimulate metabolite production inducing the release of certain hormones or neurotransmitters.

In many cases the zombifier is working with something that’s already present in the host. For example, there’s a type of wasp that targets orb-weaver spiders and seems to induce the spider to make a different type of web from their normal one – more like one they would normally make when they are moulting. The wasp’s larvae drain the spider dry and are then suspended and protected in the dead spider’s last web. The larvae seem to be tinkering with existing instructions in the spider's behavioural manual, rather than coming in with completely new instructions.

Why are arthropods so susceptible to zombification compared with other animal groups?

Arthropods are very old and extremely abundant, and parasitism is a very successful strategy. Parasitologists joke that the first form of life was free living and the second was parasitic. These are often very long evolutionary relationships, where at some point evolutionary pressures forced parasites to add behaviour manipulation to their arsenal.

Arthropods are particularly susceptible to zombification.
Tim Fogg / Gilles San Martin

There is an interesting paper that reconstructed a family tree for Ophiocordyceps to try to trace the origins of behaviour manipulation in that fungus. The scientists were able to trace all the behaviour-manipulating species to a common ancestor that infected beetles but didn’t manipulate them. It is thought that when the fungus switched to infecting ants, it was able to evolve a strategy that exploited the fact that ants are social insects. Typically, ants kill and discard the corpses of parasitised nestmates, so there was evolutionary pressure for the fungus to find a way to change the behaviour of infected ants so they would leave their colony before their sisters discovered that they were infected. The fungus manipulates the ant to find a nice, secluded spot and die there, clinging to a leaf or twig. And then the fungus later produces a stroma that disperses its spores.

In your book, you touch on concerns that people might one day be able to exploit zombifiers to manipulate or zombify humans. What’s your view on how feasible that is?

If we’re talking about, say, a zombie ant fungus potentially jumping to people – the plot at the heart of recent hit TV show The Last of Us – then no [see ‘Fact inspires fiction’, opposite page]. The evolutionary leap is just too great and in general most parasitic fungi do not survive in our warm mammalian bodies.

That being said, there are pathogens that are known to affect mammal behaviour. Obviously, rabies is something that has been around and written about for thousands of years, and Toxoplasma gondii has long been known to affect behaviour in infected rodents. Rodents are normally fearful of the smell of cat urine, but T. gondii-infected rodents are attracted to it, thought to be because cats are the parasite’s definitive hosts.

Ultimately, human behaviour is so complex, and there are so many factors that shape our behaviour, that it seems unlikely to me that there is any one chemical compound or protein that is a magic key to unlock general behaviour manipulation. It seems unlikely that there is going to be one compound that is going to suddenly compel people to move towards light, for example. As far as specific behaviour manipulation goes, evolution tends to come up with a solution to very specific problems, and the behaviours that we see manipulated are often related to moulting behaviour or metamorphosis, neither of which exist in humans.

The notion of the zombie is uniquely fascinating, because the idea of free will is something that has always fascinated people: to what extent are we in control of our own actions and how are we influenced in ways that we don't necessarily see or understand? Before pop culture, the zombie was something that originated in cultures in Western Africa, then travelled to the Americas with enslaved people, and then grew out of religious and cultural practices. I think these are conversations that people have and think about very often, even if they aren’t thinking specifically about parasitisation.

Zombifying parasites have been dubbed ‘nature’s neuroscientists’ for their ability to rewire their hosts’ nervous systems. Photo courtesy of Harry Evans

I've read theories that humans infected with T. gondii have reduced aversion to risk. Is that something you looked into in the book?

Yes, there have been a lot of studies showing that T. gondii infection affects behaviour in mammals other than rodents, including wolves, hyena cubs, chimpanzees and sea otters. In all cases, there is a type of boldness that is unusual. In chimps, there’s a loss of fear around cats, despite leopards being their natural predators.

There is a growing body of evidence that T. gondii can also change human behaviour, even if the person shows no other symptoms – things like increased confidence or risk-taking. However, there’s still a lot of work to be done on how it may manipulate humans or disrupt normal brain function. And with a lot of these studies it’s hard to say for sure that T. gondii is the only factor driving the change.

Finally, do you have a favourite example of zombification? The more elaborate or unsettling the better!

Definitely the most visually striking one is Leucochloridium, a type of worm that infects snails. As larvae, they create broodsacs in the snail that are very colourful and patterned. These then migrate into the snail’s eyes stalks, so you have a snail with translucent, stretchy eye stalks stuffed with colourful broodsacs, and the movement of the larva inside them makes a pulse. They’re sometimes called ‘disco zombie snails’, because the pattern of the pulsing is actually quite mesmerising.

Weisberger believes the evolutionary leap of a zombie fungus jumping to people is unlikely.
Photo courtesy of Harry Evans

The worms manipulate the snails to leave the undergrowth and go out into brightly lit, open areas. The pulsing, striped, colourful broodsacs in their eye stalks look an awful lot like caterpillars. This all makes them more visible and appealing to birds. 

The broodsacs detach from the snail very easily, so a bird will see this display, swoop down, snatch one up, and voilà, the worm is inside the bird's guts, where it needs to be to reproduce. Snails infected by two different species of Leucochloridium can have different coloured sacs in each eye stalk, which is really something to see.

Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control is published by Johns Hopkins University Press.

Mindy Weisberger is a science journalist and author of Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control.