# How would a virus that only kills one sex work?

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.

• 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. May 12 at 16:21
• How selective this virus needs to be? 90% accuracy, 99% accuracy, 99.99% accuracy? May 12 at 16:57
• 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.
– Tom
May 12 at 18:25
• Something for OQ to consider: what would be wrong with hormones? Testosterone bad; estrogen good? If more than that is needed, why is it? May 15 at 22:08

# 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.

• A gene to make them male. Only most of the time. May 12 at 17:41
• It's theoretically possible for a virus to be activated by binding to a certain DNA sequence. This kind of virus wouldn't arise naturally, because there's no evolutionary sense for a human virus to limit its replication by searching for a certain stretch of DNA after entering the cell. The virus would have to be deliberately engineered. The opposite scenario - deactivation by binding to target - doesn't seem possible to me. One male somatic cell contains only one Y chromosome, so many copies of a fast replicating virus would escape deactivation. May 12 at 18:14
• All but 1 in 50 000 = 'virtually all', not 'most'. May 12 at 18:57
• 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. May 12 at 19:19
• 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. May 12 at 23:27

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 feeds off of one of those, 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 feed on 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.

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.

• 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?" May 12 at 18:06
• @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. May 12 at 18:42
• And they can definitely make their host cells make toxins, which constitutes 'makes chemicals' in my book. May 12 at 18:56
• 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. May 12 at 19:04
• Or rather they make the host cells do it, but same thing May 12 at 22:33

Chivalry