Do men experience more severe symptoms than women when infected with the same virus? Recent studies have suggested that man flu is actually a thing – men are more vulnerable to the influenza virus due to having higher levels of testosterone and the fact that women are protected by estrogen, which has strong anti-viral properties. Now, researchers in the UK say that you can thank evolution for this too – viruses tend to be "kinder" to women to ensure they are passed on to their offspring.

Medical records show that the Influenza A virus is not the only one with a predilection for attacking men more severely. Men infected with TB are 1.5 times more likely to die than infected women, while those infected with Human Papillomavirus (HPV) are five times more likely to develop cancer and at least twice more likely to develop Hodgkin's lymphoma when infected with the Epstein–Barr virus.

However despite growing evidence to show that infection diseases behave differently in men and women, doctors tend to prescribe the same treatment to both sexes. One reason is that not much is understood about why viruses infect men and women differently.

Royal Holloway researchers argue that rather than just ascribing this to the differences in immune systems, attention needs to be given to the pathogens' evolutionary drive as well. Women are not affected as severely because they, especially those who are pregnant, are valuable hosts who can pass the virus on to their child either through childbirth or breastfeeding. Men, unfortunately, cannot do the same.

"Survival of the fittest is relevant to all organisms, not just animals and humans," says co-author Francisco Ubeda at Royal Holloway University of London. "Viruses may be evolving to be less dangerous to women, looking to preserve the female population. The reason why these illnesses are less virulent in women is that the virus wants to be passed from mother to child, either through breastfeeding, or just through giving birth."

In their paper, the researchers used mathematical modelling to show the role natural selection plays in lowering the fatality rate in women who can transmit the virus to children, than in men. To argue their case, they looked at a case that has long puzzled the medical community: Why is the Human T-cell Lymphotropic Virus Type 1 more likely (about 2 to 3.5 times) to become leukemia in Japanese men than women, and why does the same virus affect both sexes equally in the Caribbean? The authors of the study argue that this difference can be traced to breastfeeding – the practice is more prolonged in Japan and this could have prompted the virus to become less fatal in women. After all, it would not benefit the pathogens to incapacitate their prized hosts. According to the researchers, a similar pattern can also be found in MTB, HPV and EBV, all of which are infections that can be passed on from mother to child.

Of course, there is the possibility that other socio-cultural factors might be involved in the discrepancies highlighted by the Japan-Caribbean case. Furthermore, there is also the question of how pathogens are able to know whether they are in a man or woman, and more specifically, a pregnant one at that. The researchers admit that they do not know the answer to this yet, though it would surely open up new possibilities involving the manipulation of the cues used by viruses to reduce the severity of infections.

That said, the study does show the need for clinical trials in both sexes, rather than predominantly in men, as is the current practice, say the authors, concluding that a better understanding of what causes the differences in virus attacks could bring us closer to developing effective sex-specific medical treatments.

The results of the study were published in Nature Communications.

Source: Royal Holloway