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.
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.
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.
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.
Chivalry
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.
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.
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.
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.
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.
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.
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.
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.
