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Science Fiction & Fantasy






Endless Forms Most Horrible:
Parasites and SF

Certain wasp-like insects, which construct in the corners of the verandahs clay cells for their larvae, are very numerous in the neighbourhood of Rio. These cells they stuff full of half-dead spiders and caterpillars, which they seem wonderfully to know how to sting to that degree as to leave them paralysed but alive, until their eggs are hatched; and the larvae feed on the horrid mass of powerless, half-killed victims—a sight which has been described by an enthusiastic naturalist as curious and pleasing!

—Charles Darwin, The Voyage of The Beagle (Chapter II)

He’s running for his life—but eventually his struggle ends in vain. The alien descends on him and delivers her sting, and then another. He finds himself paralyzed after the first, so the second one can be delivered precisely into his brain. He cannot move as the sharp tip penetrates his head. Afterwards, he begins to feel contented, lethargic. The alien leads him to her lair and lays a single egg on his belly. He watches helpless as it hatches and the larva burrows into him. He feels it gnawing through his insides, but can’t do anything about it. He’s still alive for most of the time; the larva knows to leave the most vital organs for last.

It sounds like a scene from some gory SF, doesn’t it? Except this kind of thing happens quite normally in the real world—only the alien is a parasitoid wasp, and its victim a cockroach.

The real Alien

Especially among insects and most prominently wasps, there are countless parasitoid species: many that need a single host for a part of their lifetimes and then discard it dead. Some live on their hosts, some inside them; there are situations where different parasitoid species can meet in a single host, or even parasitoids using other parasitoids as their hosts. Such scenarios are common in nature and would be interesting to explore in the realm of science fiction, but “simple” parasitoid occurrences are most frequent in the genre, the best known being the cult franchise Alien. You have likely felt the horror when the almost-mature xenomorph burst from Gilbert Kane’s chest, killing him.

What are the chances we’d meet a parasitoid among alien life? I’d say that if we encounter a complex biosphere, the odds are pretty high, practically certain. On Earth, parasitic—including parasitoid—life strategies have evolved countless times independently in many lineages, and nearly half of all described species are parasites. But don’t worry: the odds of encountering a xenomorph seem next-to-nothing, since most parasites have coevolved with their hosts for a long time and are adapted only to a limited range of hosts, frequently a single species.

In any case, Alien didn’t come up with anything entirely new in the genre. Its story, atmosphere, suspense, and visuals have just made it a cult classic. But science fiction, fantasy and horror have contained the theme of parasites from the very beginning, even from their folklore and mythical roots. (Although, to be fair, vampires that don’t kill their victims aren’t technically parasites in the narrow sense but micropredators—like mosquitoes—and those who kill are simply predators.)

Parasites can be found in Victorian novels (e.g. Dracula by Bram Stoker and The Beetle by Richard Marsh, both published in 1897). Those readers knew about parasites quite well and both feared and loathed them. And newer works? We can see a lot of parasitic life strategies in them. Perhaps surprisingly, brood parasitism—common not just in cuckoos, honeyguides, cowbirds, and some other bird species, but also in other lineages of life, seems rather rare in modern speculative fiction. John Wyndham’s SF novel The Midwich Cuckoos is a prime example, but we actually see a lot of this parasitic strategy in myths and legends. Just think of elves stealing human babies and replacing them with their own kin.

Pulp and Golden Age SF seems rife with parasites of various kinds. The space being Ixtl in A. E. van Vogt’s The Voyage of The Space Beagle is a parasitoid, laying eggs into the bellies of several crew members to eat them from the inside. Heinlein’s The Puppet Masters introduce mind-controlling sluglike creatures who attempt to take over humanity, albeit with somewhat unclear strategy as what to do next or how their plan maximizes their fitness. But that’s the trouble with SF parasites that possess intelligence: they tend not to reach the success, or the horror effect, of their less endowed cousins.

In Cronenberg’s movie Shivers (1975), a parasite engineered by a mad scientist isn’t intelligent, or at least doesn’t seem so. It increases its hosts’ sexual desire to the point where they become complete slaves to it. The parasite is mostly—though not solely—transmitted during sex. A similar strategy is suspected to be used by a number of real parasites, but there is surprisingly little solid evidence for it, perhaps because there are many host behavioral defenses to be overcome (such as “don’t mate with someone who seems sick”). Even if observed, it might be just because a sick host, uncertain of their future reproductive prospects, attempts to go “all in” in the game of reproduction.

Some parasites, though, manage to make their host attractive . . . even if it’s already dead. The parasitic fungus Entomophthora muscae first controls its host, a house fly, so that it flies to a high point and spreads its wings and legs so that the spores of the fungus that has grown through its entire body can be easily caught by wind and infect other flies. But that’s not all: Infected (and dead) female flies become attractive for male flies, who can become infected by attempting to mate with the cadaver. Sounds gross? It certainly does; but “irresistible corpses” may be difficult to pull off in any fiction.

Mind control and body-snatchers

But what draws perhaps more attention and fear than such gory images is exactly the issue of mind-controlling parasites. After all, the idea of something or someone else seizing control of our selves seems appalling at least to most people. Be it the above-mentioned examples, Goa’uld in Stargate, Ceti eels and unnamed “parasitic beings” in Star Trek, Bezoar in Buffy The Vampire Slayer, headcrabs in Half-Life, they terrify us if executed well. Yet in reality, we are very much “the sum of our parts” consisting also of organisms other than human. The myriad of microorganisms—various protists, bacteria, archaea, viruses that are living inside our bodies, many of them practically indispensable for us, all affect not just our physical health, but also our mental states. Research on this so-called microbiome is still quite young, but there are numerous indications that the composition of our gut microbiome is linked to mood and behavior, especially stress response. On the other hand, we must be careful to correctly determine the cause and effect there. It’s likely that our gut microbiome influences our moods, but also that our behavioral patterns impact the composition of microbes in our gut (because of our eating, exercising, and other habits). Which direction is the major one in this two-way relationship? That remains to be investigated more in-depth.

But most of the organisms in our gut are either commensals, or mutualists. They either don’t do much good or bad for us and basically just consume the scraps that we don’t want anymore, or they have a positive effect on our health. In truth, you’d never find a perfect commensal as their presence always has some effect and even mutualists can “turn bad” under special circumstances, but we can generalize and say that their presence is neutral or good. Our bodies are used to them and need many of them. We don’t think of their influence over our mood and behavior as manipulation. But what about real parasites?

There are plenty of real-world examples of host manipulation. For instance, the rabies virus increases migratory behavior and aggressiveness in infected animals, increasing the chances for its long-distance transmission (by the way, outbreaks may have paved the way for werewolf trials and legends). The liver fluke Dicrocoelium dendriticum makes its second intermediate host, an ant, climb a grass blade and hang from it on its mandibles, so that the definitive host—a ruminant—can eat it. Caterpillars infected with baculoviruses seek high places in trees so that virions released from their dead bodies can reach more new victims. Hairworm-infected crickets jump into water when the parasite is mature, and drown while the worm crawls out of their bodies and happily swims away. The parasitic crustacean Sacculina (used fictitiously in Philip Fracassi’s horror novella of the same name) not only castrates its crab hosts, but makes them take care of the spreading of its own progeny. Malaria-infected mosquitoes carrying infectious stages of the protozoan are more attracted to humans and tend to bite more people (though in this case, the parasite more likely co-opted the sickness response of the host rather than manipulating it outright).

Perhaps most famously, the malaria-related protozoan Toxoplasma gondii manipulates its intermediate hosts—typically rodents—to become easier prey to cats. They exhibit less fear (and cease to be afraid of cat odor, even become fatally attracted to it), are more active, and react more slowly. Several studies suggest that infected humans are more likely to fall prey to . . . cars. Slower reaction times and more risking tendencies are among the commonly observed effects in both rodents and humans, who are mostly “dead ends” for T. gondii these days. But how does the parasite achieve this? It seems that the protozoan influences its hosts’ dopamine metabolism and thus behavior. Toxoplasma-infected individuals also display higher testosterone levels—decreasing their immune response and shifting behavior toward more risk-seeking and aggression (in male subjects) at once.

If more zombie stories were inspired by these real-world parasites, perhaps there would be much more variety to them. And indeed, there are examples of brilliantly executed stories using such themes: George R. R. Martin’s Men of Greywater Station uses the trope of a parasitic fungus—except in the story, it has intelligence and agency, and manages to brilliantly manipulate not just its victims, but also the humans it hasn’t infected yet. M. R. Carey’s acclaimed novel The Girl with All The Gifts even uses a real fungal parasite, Ophiocordyceps unilateralis (which infects and “zombifies” insects in reality), as the source of an unusual zombie outbreak.

Note, however, that most behavior-manipulating parasites have more complex life cycles with at least one intermediate host, sometimes many of them and that’s something we don’t often see in science fiction. This trope might prove more interesting yet.

Parasites: loathed but indispensable . . . and wonderful

It might seem that parasites are horrible creatures that should be exterminated, but nothing can be further from the truth! Even if we set aside the ethical issues of causing a species’ extinction, even though that species is considered harmful, many food webs are kept in delicate balance by some key species! Parasites often represent this linchpin, for example, by keeping a predator population in check and preventing it from growing too much, eliminating its prey and causing a whole domino effect in a given ecosystem. Sometimes, if we recognize the key role of a parasite, we can use it to limit populations of invasive species in an environment where the species was introduced without this natural regulation. It doesn’t always work, but for example the parasitoid wasp Trissolcus basalis has proven successful in many regions where its host, the southern green stink bug, previously wreaked havoc on crops.

Still, this probably wouldn’t make you willingly swallow a parasite, right? Despite that, there are reasons why you might want to do just that. If you have allergies or certain autoimmune disorders, exposure to parasites can actually alleviate your trouble. A parasitic worm that infects pigs, Trichuris suis, weakens the symptoms of Crohn’s disease (probably by altering the gut microbiome) and presents no significant danger for humans. This kind of helminthic therapy is also used for other inflammatory gut diseases, and we may one day treat and try to prevent allergies with parasite-derived proteins. These trends have already made their way into science fiction: Mira Grant’s Parasitology series uses a similar premise, in this case an engineered tapeworm enhancing the human immune system and other functions, for its SF-horror plot.

In terms of mood and behavior alteration, parasites might also be utilized deliberately. A slightly ironic docufiction article on The Verge recently explored the idea of using T. gondii voluntarily in the competitive, aggressive environment of Silicon Valley. Another instance where T. gondii occurred as a remedy of sorts was a mention in Peter Watts’ novel Echopraxia. In reality, people are talking about changing not just their health but also their behavioral patterns through microbiome fecal transplants, and though the science is quite new, there will undoubtedly be many amateurs and biohackers experimenting with such new treatments . . . and a more than fertile ground for science fiction authors.

We tend to be horrified and disgusted by parasites and some of us wish to eliminate them completely. But in many ways, we depend on them. Food webs can hinge on them and they present an important evolutionary force. They might even offer avenues into medical research. Moreover, they are simply too interesting.

We still seek to understand the arms race they engage in with their hosts’ immunity; how they manipulate hosts’ behavior, both indirectly and directly. From producing neurotransmitters to utilizing symbiotic viruses. We have yet to learn the extent of the vital role they play in Earth’s biosphere. It is presumed that the majority of species on Earth are or can facultatively become parasitic. Some are deadly; some effect their death toll only through minute changes in each host, outright killing none; some are evolving into helpful mutualists.

If you want to understand ecology and evolution, you can never leave out parasites. Thinking about them this way, as incredible and all-pervading creatures, you can perhaps finally understand the sentiment of Ash from Alien, saying with awe: “You still don't understand what you’re dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility.”

Note/disclaimer: The author works in the department led by Professor Jaroslav Flegr, who is among the leading researchers on Toxoplasma gondii’s behavioral effects in humans (though the author doesn’t work on this topic herself). If you’re interested in this research or helping science in general by participating in various (mostly online) studies, please consider joining the Lab Bunnies community at

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ISSUE 145, October 2018

Best Science Fiction of the Year


Not One of Us


Julie Novakova

Julie Novakova is a Czech author and translator of science fiction, fantasy and detective stories. She has published short fiction in Clarkesworld, Asimov's, Analog, and other magazines and anthologies, and her translations of other authors' work appeared in, Strange Horizons, and F&SF. Her work in Czech includes seven novels, one anthology (Terra Nullius), and over thirty short stories and novelettes. Some of her works have been also translated into Chinese, Romanian, Estonian, German and Filipino. She received the Encouragement Award of the European science fiction and fantasy society in 2013, the Aeronautilus award for the best Czech short story of 2014 and 2015, and for the best novel of 2015. Julie is an evolutionary biologist by study and also takes a keen interest in planetary science.


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