Why men might have a point about ‘man flu’ – viruses want to kill men more than women, scientists find

Historically men have been mocked for their inability to handle even mild viruses, with the term ‘man flu’ often used to describe the male experience of the common cold.

But a new study suggests men might have a point. Some viruses really are out to get them.

Researchers at Royal Holloway University have discovered that certain viral infections have evolved to be more virulent in men.

They appear to be particularly nasty if they are the sort of virus that is transmitted from mother to child, such as rubella, chickenpox, zika and hepatitis.

Put simply, women are more valuable to the virus than men are because they can pass it on to more people.

"Viruses may be evolving to be less dangerous to women, looking to preserve the female population,” said Dr Francisco Úbeda, of the School of Biological Sciences at Royal Holloway.

“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."

Researchers looked at the virus Human T-cell Lymphotropic Virus Type 1 (HTLV-1), which can cause leukaemia in infected individuals.

Infected women tend to develop leukaemia less often than men when there is more mother-to-child transmission.  

Death due to infectious diseases is often higher in men than in women, but it has previously been attributed to differences in the immune system of each sex.

The study suggests it is the virus itself which prevents women becoming too ill.

"It has already been established that men and women react to illness differently, but evidence shows that viruses themselves have evolved to affect the sexes differently," said Professor Vincent Jansen, from the School of Biological Sciences at Royal Holloway, University of London.

The researchers used mathematical modelling to show that natural selection favours viruses that have a lower rate of fatality in women than in men, if the virus can be passed from person to person and from mother to child, either in childbirth, breast-feeding, or close contact in infancy.

They also looking at how HTLV-1 affects people in Japan and the Caribbean. The research showed that HTLV-1 is about 2 to 3.5 times more likely to progress to become Adult T-cell Leukaemia (ATL), which is lethal, in Japanese men than women.

In the Caribbean, however, the likelihood of HTLV-1 progressing to leukaemia is roughly equal in men and women.

The researchers believe that because breastfeeding is more prolonged in Japan, giving more opportunity for it to be passed onto offspring, the HTLV-1 virus has evolved to become less fatal to women than in the Caribbean where breastfeeding is shorter.

Women are more valuable as hosts for the pathogens when they are able to pass on the pathogen in more ways than men, who are only capable of transmission from person to person.

"Pathogens are adapting to be less virulent in women to increase their chances of being passed on to the next generation during pregnancy, birth and infancy," added Dr Úbeda.

"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."

The research was published in the journal Nature Communications.