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There is a popular movie trope of the near-total elimination of a sex (often male) because of a virus/a plague. This got me wondering :

How would such a virus work?

I'm not sure if it would be possible, but I had one idea: the virus would mainly attack those with a high amount of testosterone, thus making men the only ones infected.

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  • $\begingroup$ Can you be more specific about what you mean by how a virus works? Is it sufficient to just say "This virus only kills men"? As a general rule, when worldbuilding extraneous explanation detracts from the quality of your world, by forcing you to commit to the consequences of said unnecessary explanation. $\endgroup$
    – sphennings
    May 12, 2022 at 16:21
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    $\begingroup$ How selective this virus needs to be? 90% accuracy, 99% accuracy, 99.99% accuracy? $\endgroup$
    – Alexander
    May 12, 2022 at 16:57
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    $\begingroup$ Something for answerers to consider: OP wants a virus that successfully kills one gender. OP does not state that both genders cannot be infected, or even that both genders can't experience serious symptoms, only that one gender disproportionately fails to survive. $\endgroup$
    – Tom
    May 12, 2022 at 18:25
  • $\begingroup$ Something for OQ to consider: what would be wrong with hormones? Testosterone bad; estrogen good? If more than that is needed, why is it? $\endgroup$ May 15, 2022 at 22:08
  • $\begingroup$ No. Such a virus would be too powerful. It would either completely destroy a population or leave a small number of uninfected. Either way, the virus would no longer exist. $\endgroup$ Mar 8 at 14:10

9 Answers 9

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Have a DNA virus.

Y chromosome holders have a gene to make them male so have a virus that binds to this, and produces a toxin that kills the host if the gene is present, or isn't present.

It's simple, if immoral, piece of genetic engineering.

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    $\begingroup$ A gene to make them male. Only most of the time. $\endgroup$
    – jdunlop
    May 12, 2022 at 17:41
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    $\begingroup$ All but 1 in 50 000 = 'virtually all', not 'most'. $\endgroup$
    – user86462
    May 12, 2022 at 18:57
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    $\begingroup$ Any biological agent runs the risk of someone being immune. For instance, there are a literal handful of people known on the planet to have survived rabies despite rabies, once symptoms appear, being essentially 100% fatal. Just because it's not technically 100% fatal and only 99.9999% fatal doesn't mean it can't serve it's purpose. $\endgroup$ May 12, 2022 at 19:19
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    $\begingroup$ I said that the virus produces a toxin based off if the gene is present. It can infect everyone, but just by chance produce a toxin if the gene is present, or isn't present. And yeah, unless it's a magical virus it's not gonna be perfectly successful. 99.9999% is pretty good though. $\endgroup$
    – Nepene Nep
    May 12, 2022 at 23:27
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    $\begingroup$ Doesn't need to be a virus. There are bacteria that do this to moths and butterflies, kinda. They don't kill one sex, but they force most (if not all) offspring of an infected insect to be of one gender. $\endgroup$ Feb 18 at 3:53
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There are two simple cases to consider: a virus that preys upon a characteristic present in only one gender, and a virus that is counteracted by a characteristic present in only one gender.

In the first case, you have viruses that target one specific gender by, for example, starting cancer in reproductive organs. HPV can lead to such cancer, but affects both genders. It's not hard to imagine a virus that only leads to cancer in one, however, maybe even a variant of HPV.

In the second case, you have viruses that are vulnerable to the defenses of only one gender. This article, for example, details differences in the immune responses of men and women. With a higher immune response, women can fight off certain diseases easier, but can be prone to autoimmune responses as a consequence.

If you're dreaming up the world, you get to set the exact mechanism by which the virus works, but these two avenues are where I'd start looking. It's a bit easier to explain a virus disproportionally affecting one gender than one only affecting one gender, but you could pull it off with a virus that triggers fast-spreading cancer that must start in reproductive organs, or maybe one that triggers an autoimmune response, in certain cases.

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  • $\begingroup$ One other problem is viruses have a strong tendency to mutate during replication. This process is an adaption that makes it easier to spread to and replicate in new hosts. Also and as a general rule the more genetically distinct from one another two species are the harder it is for a virus to jump the species barrier. In the case of a virus that just had to jump the gender barrier? I doubt it would take more than a few generations/iterations of the virus before the jump could be made. $\endgroup$
    – Mon
    Feb 14 at 3:18
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This is painfully easy if you're targeting men, but not reliable. All viruses target specific proteins. There are numerous proteins that are only expressed by the Y chromosome. If the virus uses those proteins to get through a cell wall, then it would only effect men.

If you're targeting only women, my understanding is that this would be much more difficult. You would have to target a protein that exists in all people, but have men express a protein that inhibits it.

The hard part would be making it infectious. You'd need it to first target an exposed protein, like in the upper respiratory system, or as a sexually transmitted disease. If it's sexually transmitted, then men would have to get it from other men. From there it would work its way through the system until it found the protein it was targeted to attack exponentially.

The unreliable part is that there aren't many proteins that are in "all men and no women." There will always be some cases where people are naturally immune, even if they do have the protein. The diseases that target those proteins are generally limited to the reproductive organs.

What you'd wind up with is something like super-syphilis. Although the mechanism isn't understood, syphilis has a higher fatality rate among men. If you could isolate that mechanism and exploit it, you might have what you're looking for.

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  • $\begingroup$ Viruses attack nucleic acids (DNA/RNA), not proteins. $\endgroup$
    – Nosajimiki
    Mar 8 at 21:59
  • $\begingroup$ @Nosajimiki, I wish it were that simple. What we can say is that viruses are nucleic acids. They have to find appropriate carbohydrates or proteins on the cell's membrane in order to attack it. Once inside, they use the cell's own replication mechanisms (which are mostly nucleic acids) in order to create more of themselves. Which part is the attack? All of it, really. My assertion was that it is the outer coating of the cell that will determine if the virus will latch on and attempt to gain entry. $\endgroup$ Mar 9 at 22:56
  • $\begingroup$ I see your point, but your wording is misleading in a few places. They don't "feed" on proteins, but they do bind to them. As for all the stuff about Women being more resistant to certain infections, that has nothing to do with proteins. Woman have unique proteins too. It is because there are certain immune responses that are female specific that make them less susceptible to certain infections. But what they are more resistant to is typically bacteria (like syphilis). $\endgroup$
    – Nosajimiki
    Mar 11 at 13:35
  • $\begingroup$ @Nosajimiki, Thank you for pointing this out. I've adjusted the answer to suit. My point about women being harder to attack relates to the reality that all men have X chromosomes, but no women have Y chromosomes. You can get into the finer details of the chemical biases generated by the expression of the genetics, but that's what I meant by "more difficult". $\endgroup$ Mar 11 at 16:06
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Chivalry

women and children first

https://interestingengineering.com/the-origin-of-woman-and-children-first-the-sinking-of-hms-birkenhead

Your people know the virus is bad. It is an existential threat and it could wipe them out. There is a treatment but there is not nearly enough for everyone.

The society makes a decision: the people who are most important are women and girls. Those are the people who get the treatment. They are the people who survive.

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    $\begingroup$ since the people in power are disproportionally male, and lacking in ethics, this seems unlikely. $\endgroup$
    – ths
    Mar 9 at 12:16
  • $\begingroup$ It could make sense as a strategy to repopulate the Earth once the virus has run its course, but only if some males are left. $\endgroup$
    – Batperson
    Mar 11 at 3:16
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There are viruses and bacteria that kinda do that in real life. The reason the species they infect still exist is because either there are always some survivors, or in some cases the species evolves to not need males at all. A bit of evil genetic engineering in a sci-fi or horror setting can do the trick of eliminating a human gender completely for you.

Meet Wolbachia the bacterium.. Though it is not a virus, you could have a virus that copies its methods. See my emphasis on the third method below:

These bacteria can infect many different types of organs, but are most notable for the infections of the testes and ovaries of their hosts. Wolbachia species are ubiquitous in mature eggs, but not mature sperm. Only infected females, therefore, pass the infection on to their offspring. Wolbachia bacteria maximize their spread by significantly altering the reproductive capabilities of their hosts, with four different phenotypes:

  • Male killing occurs when infected males die during larval development, which increases the rate of born, infected females.
  • Feminization results in infected males that develop as females or infertile pseudofemales. This is especially prevalent in Lepidoptera species such as the adzuki bean borer (Ostrinia scapulalis).
  • Parthenogenesis is reproduction of infected females without males. Some scientists have suggested that parthenogenesis may always be attributable to the effects of Wolbachia, though this is not the case for the marbled crayfish. An example of parthenogenesis induced by presence of Wolbachia are some species within the Trichogramma parasitoid wasp genus, which have evolved to procreate without males due to the presence of Wolbachia. Males are rare in this genus of wasp, possibly because many have been killed by that same strain of Wolbachia.
  • Cytoplasmic incompatibility is the inability of Wolbachia-infected males to successfully reproduce with uninfected females or females infected with another Wolbachia strain. This reduces the reproductive success of those uninfected females and therefore promotes the infecting strain. In the cytoplasmic incompatibility mechanism, Wolbachia interferes with the parental chromosomes during the first mitotic divisions to the extent that they can no longer divide in synchrony.
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The virus can lie dormant in the male urethra or female uterus. It can only reproduce in one of the two, depending on whether it targets men or women. It spreads through sexual contact. Once it gets where it wants to be it wakes up and starts replicating. It produces a chemical that spreads through the body and poisons it.

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    $\begingroup$ Viruses... do not "produce chemicals". They're not sophisticated enough for that. Pretty much all of their tiny genome is made up of "how do I make more of me?" $\endgroup$
    – jdunlop
    May 12, 2022 at 18:06
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    $\begingroup$ @jdunlop Not all viruses are small. The largest known ones (Mega- and Mimiviruses) have genomes of over 1 million bases that encode for more than 1000 proteins. Wikipedia knows of only one viral toxin, but there's no reason why a virus could not be engineered to produce something nasty. $\endgroup$
    – Cloudberry
    May 12, 2022 at 18:42
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    $\begingroup$ And they can definitely make their host cells make toxins, which constitutes 'makes chemicals' in my book. $\endgroup$
    – user86462
    May 12, 2022 at 18:56
  • $\begingroup$ The problem with a sexually transmitted virus that only infects one sex is that it spreads only among individuals with homosexual contacts. To spread through heterosexual contact, it has to infect both sexes, and toxin production must be activated separately of replication. Then we again have the problem of how to make it sex-specific. A deadly STD will also be noticed long before it has had a chance to spread through the whole population, and people will protect themselves. $\endgroup$
    – Cloudberry
    May 12, 2022 at 19:04
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    $\begingroup$ Or rather they make the host cells do it, but same thing $\endgroup$
    – LWS SWL
    May 12, 2022 at 22:33
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Women are more susceptible to autoimmune diseases and about 80% of people who have an autoimmune disease (examples include type 1 diabetes and rheumatoid arthritis) are women. Recent (2024) research seems to make the claim that since women have 2 X Chromosomes, one of which has to be turned off as the blastocyte develops, and errors in this mechanism are why women have much higher rates of autoimmune diseases. A recent study (Press release, simplified newspaper version) claims a molecule named Xist, which only exists in women, is responsible for turning one chromosome off.

Your hypothetical virus attacks Xist, or the mechanism by which Xist operates, and causes cells with 2 X Chromosomes to develop antibodies to themselves and kill themselves off. If this virus affects other mammals, it will probably kill off all female mammals because mammals use an X-Y sex determination scheme (where females have 2 X chromosomes and males have one X and one with a broken leg that looks like the letter Y). Perhaps, in your story, this jumps species from birds, which use a different (called a ZW scheme) where males have both sex chromosomes. Male birds have ZZ for their sex chromosomes, while female birds have ZW chromosomes - they are not shaped like the letters, those letters are used to distinguish them from the X & Y chromosomes of mammals (you can feel safe though, no genes on X/Y chromosomes match with genes on Z/W chromosomes).

The Screwfly Solution (the short story) uses the premise that the aliens dose Earth with a pesticide that makes men ultraviolent towards women, children and "crypto-women" (which I suspect were effeminate men). The Masters of Horror video adaptation says that it was a virus. I think this story implies that civilization is a thin veneer covering a dark murderous "natural man" - all the aliens had to do was to peel back that layer protecting civilization, or "turn off" our domestication.

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A virus close to meeting your requirements, in that it preferentially kills one biological sex over the other, already exists, and something like 10-20% of the population may already be infected: Human papillomavirus (HPV).

HPV itself isn't that nasty. It causes warts and scars and most people recover fairly quickly. The problem is that many of these people go on to develop cancer, most commonly cervical cancer. The virus can also cause penile and anal cancer in men, but this is far less common.

This gender disparity has very real-world effects, and is the reason women are far more likely to receive an HPV-vaccine than men.

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It is easier to target women than men with a virus

Viruses are not actually unique organisms, but rather, the result of broken strips of the host species's own DNA wrapped in proteins that behaves maliciously towards the host when separated from the rest of the chromosome it replicates from.

Male and Female DNA is identical except for the X/Y chromosome. Male DNA is missing a piece of genetic code that Female DNA has; so, if a virus were to require something from this missing section of DNA, then it would be 100% harmless to men, but would attack women.

But a retrovirus could attack only men

The vast majority of viruses attack DNA, but there is a small subset called retroviruses that specifically target RNA. Because males and females both have unique forms of RNA, it is theoretically possible for a retrovirus to attack just male RNA. That said, retroviruses are far less common in humans than DNA viruses. Out of the 219 known viruses that affect humans, only 5 are retroviruses, and all of them affect women more than they affect men. So, male specific viruses are theoretically possible, but nature tends to swing in the other direction.

A Male specific Bacteria, Fungus, or Parasite is much more plausible

There are enough minor differences in males and females that an actual organism would be far more apt to thrive in a host of one gender or the other. Again, most infectious diseases affect both genders pretty equally; however, there are certain immune responses that exist wholly on the X chromosome making the female immune system is a bit more robust over all. So, while it is unlikely for a disease to just affect men, it is very possible that a particularly bad disease could do something like kill 99% of men and only 95% of women leaving a highly gender skewed population behind.

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