Viruses, it turns out, have most likely evolved to affect men in more severe ways than women: tuberculosis, papillomavirus, and others have that in common. Now a fresh study has discovered the surprising reason why that might be.
And it’s not because pathogens are sexist. Part of the explanation could be the differences in the two sexes' immune systems. But scientists at the Royal Holloway University of London believe they've made a breakthrough in identifying the main reason - that women are more valuable hosts for the viruses.
The differences in severity are stark. As it turns out, men are 1.5 times more likely to die of tuberculosis than women, twice more likely to develop Hodgkin's lymphoma from the Epstein-Barr virus as well – and a whopping five times more likely to develop cancer if infected with the papillomavirus.
As the researchers point out, we may have the wrong idea about how a virus spreads and what really drives it. The answer, they say, lies in women’s capacity to create life: they’re simply more valuable to the virus than men.
The aim of a virus is not so much to infect, as it is to replicate itself as many times and within as many hosts as possible. Women win hands down here: with their ability to get pregnant, they inherit several ways of spreading a virus that men do not – from being pregnant to breastfeeding, even during birth itself.
“Viruses may be evolving to be less dangerous to women, looking to preserve the female population,” Dr Francisco Úbeda, a biologist at Royal Holloway, confirms in the press release.
Like other organisms, viruses are subject to natural selection. By using mathematical models, the Royal Holloway scientists posit that virus survival is more likely to happen in the less infected host, especially when the host is pregnant. This is aptly demonstrated in the case of the T-cell lymphotropic virus type 1 (HTLV-1), which has been shown to affect men in Japan more than either sex in the Caribbean.
Why is that? Because traditionally, the Japanese breastfeed for longer, making it a more safe passage from the mother to infant. The virus does not want to infect the woman as much as it wants to be transmitted to the next host.
Where it gets interesting is the mechanism by which the virus knows whether it is inside a man or a woman. Vincent Jansen of Royal Holloway tells New Scientist that this is where continuing research should lie, as “there are all sorts of hormonal and other pathways that are slightly different between men and women.” And we can only manipulate virus behavior if we understand the mechanism.
“We could try to make the virus think it’s in a female body rather than a male body and therefore take a different course of action,” Jansen adds.
All of this is replicated in animals as well, with chicken populations showing greater propensity for male illnesses than female ones, according to Jansen, who wants research to continue with animals, where that particular sex dynamic has not been studied.
“Survival of the fittest is relevant to all organisms, not just animals and humans. It's entirely probable that this sex-specific virulent behaviour is happening to many other pathogens causing diseases. It's an excellent example of what evolutionary analysis can do for medicine,” Úbeda adds in the press release.