Some viruses have evolved to affect men more greatly than women

Posted Dec 15, 2016 by Tim Sandle
Is man-flu real? New evidence suggests that some viruses cause weaker symptoms in women than in men. The reason is evolutionary, based on helping the virus to spread between hosts.
Man-flu is a topic that enhances the gender divide: how men and women react to contracting a viral infection like the common cold where, perhaps stereotypically, the man takes to his bed while the woman soldiers on. Is there any truth to this biologically? Perhaps, according to new research from Francisco Úbeda and Vincent Jansen at Royal Holloway University of London. Talking with The Daily Telegraph, Francisco Úbeda said: "Viruses may be evolving to be less dangerous to women, looking to preserve the female population."
The researchers provide evidence that there is a difference between the immune systems of men and women and that, with viral infections in particular, the symptoms are often weaker with women. This is not simply because women are from a tougher stock but a face of the way the virus infects the host. For some viruses, women are of greater value given the role of the virus to replicate and to infect other hosts. Since women can pass infections to their children more readily (be that at birth or breastfeeding, for example), an evolutionary pressure may have developed for viruses to be less harmful to women in order to increase the chance of the virus being passed on to a new host.
If a virus makes someone too ill the chances are they will retreat into isolation (or at least decrease their contract with others); with this regard men are less important to some viruses than women. So, the man can become more sickly and retreat to bed whereas the woman can continue with her tasks.
The researchers, according to New Scientist, discuss the evolutionary pressure and the infectivity of viruses around a case study: human T-cell lymphotropic virus type 1 (HTLV-1). This virus progresses to leukemia more often in Japanese men compared with Japanese women; however, in the Caribbean both men and women are affected equally. The reason is, the scientists argue, is because breastfeeding rates are considerably higher among women in Japan than they are in most Caribbean islands. HTLV-1 has therefore adapted to one geographical region differently to another. How exactly a virus differentiates the gender of a host is uncertain, but it may relate to the presence of different hormones.
The new research has been published in the journal Nature Communications, with the article titled "The evolution of sex-specific virulence in infectious diseases."