Do diseases impact one sex over another, like in Chaos Walking or Y: The Last Man?

We all know how scary a pandemic can be. A deadly disease is ever-present, invisible, and frightening. In fiction, such plagues can become a bit stale as an inciting incident unless a writer does something interesting with it. Maybe the virus comes from an unusual place, like a downed satellite (Andromeda Strain) or a passing celestial body (Night of the Comet). Or maybe it only affects a specific sex. This notion was famously explored in the Vertigo Comics series Y: The Last Man (2002 - 2008) by Brian K. Vaughan and Pia Guerra. There, a global event wipes out every mammal with a Y chromosome, save for one dude and his pet monkey.

More recently, the movie Chaos Walking, based on Patrick Ness’ 2008 novel series of the same name, hinges on a similar notion.

**SPOILER WARNING: Spoilers for Chaos Walking below.**

The characters in Chaos Walking live in a world without women, all of whom were supposedly killed by a disease that targeted them specifically. The truth is a little different.

The men of this world are impacted by “the Noise,” a disease that reveals their thoughts as perceivable sights and sounds. Women, for some reason, aren’t affected. Over time, the Noise drove the men of one town to madness, resulting in the mass murder of all the women who lived there.

Chaos Walking's disease might not have killed one sex, but it still only affected men. Is there any truth to this idea that a disease could only affect men while women are immune, or vice versa?

THE OBVIOUS

Human chromosomal configurations are complex, nebulous things that don’t always correlate to associated organs or tissues. Typically we talk about XY in males and XX in females, but those are far from the only possibilities. Any sex-specific discussion must be undertaken with that in mind.

It’s also worth noting that research has historically had a binary bias and likely doesn’t tell the whole story of the ways in which disease behaves differently across sex chromosome variations. We can’t (and won’t attempt to) make any black and white conclusions, but we can talk in generalities based on the available evidence, bearing in mind that those generalities are only part of the whole picture, both in the real world and in fictional world-building. Additionally, while the author notes that the terms “male” and “female” are limiting and not fully inclusive, they will be used herein to refer to individuals with XY and XX sex chromosomes (or non-human equivalent), respectively.

There are, of course, diseases that require sex-specific organs and as such, impact those individuals who possess those organs. For instance, testicular cancer impacts the testes. So it stands to reason that it favors one part of the population. Likewise, breast cancer has a majority impact on females, though it’s not unheard of in males.

Human cancers, however, aren’t contagious and wouldn’t spread across a population. They are prevalent, devastating, and worthy of serious research and attention, but they don’t make for good post-apocalyptic genre-fiction catalysts. What we need is a communicable or genetic disease that favors one side of the population.

SEX-SPECIFIC VIRULENCE

There is evidence that pathogens impact the sexes in different ways. In some species of insects, there are bacteria that, when passed onto the next generation, selectively kill the males. There is also evidence that pathogens express differentiated virulence across sexes depending on if the mode of transmission is horizontal (among individuals of the same generation) or vertical (from mother to child).

Pathogens that transmit vertically tend to impact males of a species more heavily than females. One explanation for this is that the pathogen trades a lower virulence among females in order to reinforce the opportunity to be passed along.

A pathogen that kills females or otherwise makes them unable to procreate cuts off the bridge that would pass them to the next generation. By dialing down the intensity among females, the disease has an increased chance of spreading.

There may be other factors at play here, as well. There is reason to believe that, on average, females have stronger immune systems than males. As such, they are more likely to fight off disease more quickly, limiting its impact.

Additionally, males tend to undertake riskier behaviors, resulting in a higher incidence of underlying factors that exacerbate disease. Males are also more likely to postpone treatment, allowing a disease to progress to a point that is more difficult to treat. All of this contributes to an overall higher rate of disease among males, though some of that is certainly more social than biological.

There are diseases or disorders that impact heavily along sex-specific lines, though thankfully they aren’t common.

Rett Syndrome almost only affects females, with an incidence rate of approximately 1 in 10,000. Rett is a developmental disorder characterized by loss of coordination and ability to communicate, among others, and is caused by a mutation in the MeCP2 gene found on the X chromosome. The reason for the bias toward females is two-fold. First, the mutation appears to be more prevalent in X chromosomes from sperm cells. Males don’t inherit an X chromosome from their male parent, and so they avoid this particular mutation when present. Females, however, inherit an X chromosome from both parents. It is possible for male children to inherit a mutated copy of MeCP2 from the mother’s X chromosome, but because they don’t have a functional copy of MeCP2, such a mutation is most often fatal before or shortly after birth.

Alport Syndrome also impacts the X chromosome but demonstrates an opposite behavior, primarily impacting males. Caused by a mutation of the gene COL4A5, Alport Syndrome affects the production of collagen in the kidneys, eyes, and ears. Because females have another X chromosome, they suffer fewer or less severe symptoms. Affected males typically suffer hearing loss followed by kidney failure.

While we do see some variance in disease virulence along sex-specific lines, there is no pathogen currently known that presents a global threat to any specific sex. Mostly, that’s because pathogens want what all things want, whether they know it or not: to pass on their genetic materials and spread. Throwing all your evolutionary chips on one space and then wiping that space out is a surefire route to extinction.

The persistence of a pathogen is an evolutionary tightrope walk that tends to ensure that, with a few exceptions and some slight variation, each of us is affected more or less equally.