The truth about ‘man flu’

Some viruses may adapt to kill fewer women because mothers can spread disease farther and wider than men.

Francisco Úbeda and Vincent Jansen from the Royal Holloway University of London in the UK found in cases where women can transmit a microbe through birth or breastfeeding, they are less likely to develop severe forms of a disease than men.

The work, they write in Nature Communications, might “bring us closer to implementing sex-specific medical treatments”.

Fatality rates of infectious diseases are often higher in men than women. This is usually attributed to a stronger immune response in women, thanks to their higher oestrogen levels.

But Úbeda and Jansen suggest the reasons might also lie with the pathogens.

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Women pass infections to surrounding populations the same ways men do – through mucous or blood, for instance – but they can also transmit some diseases to their children during pregnancy, birth and breastfeeding.

These extra transmission routes can cause evolutionary pressure for pathogens to treat men and women differently. Pathogens that don’t kill their female host can, then, be passed on more effectively.

Úbeda and Jansen 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.

This was backed up an epidemiological study of a retrovirus, called human T-cell lymphotropic virus type 1 or HTLV-1, in populations from Japan and the Caribbean.

HTLV-1 is sexually transmitted, but is also passed to offspring through breastfeeding. Either way, it can progress into the lethal adult T-cell leukaemia.

In Japan – where breastfeeding is more common so the pathogen is often transmitted this way – men have more than double the chance of developing the life-threatening leukaemia than women.

But in the Caribbean, where the pathogen is less often passed through breastfeeding, men and women are equally likely to develop the severe form of the disease.

Although interesting, these results might be considered controversial by epidemiologists, says Vitali Sintchenko, a public health microbiologist at the University of Sydney, Australia and who was not involved with the study.

HTLV-1 is “a very uncommon disease to gain high quality estimates for a model”, he explains, “and the documented differences in infection outcomes between sexes are not large, in practice”.