Women sometimes complain that when the men in their lives get the flu, they get to stay in bed, whimper about their symptoms and have someone take care of them. Whereas if women get sick, we carry on regardless — after all, the family needs food, laundry needs washing and the kids need, well, everything. But while we women pride ourselves on our ability to function even with the flu, a new study shows the guys aren't exaggerating after all.

Research from Royal Holloway University of London shows that viral infections can evolve to affect men worse than they affect women because the viruses consider women to be more valuable hosts because women more easily spread viruses to children. In other words, the so-called "man flu" is real.

"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," Vincent Jansen, a professor from the School of Biological Sciences at Royal Holloway, said in a news release.

"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," added Dr. Francisco Úbeda, also from the School of Biological Sciences at Royal Holloway.

Jansen and Úbeda used mathematical modeling to show that natural selection favors viruses with a lower rate of fatality in women than in men — so long as the virus can be passed from person to person and mother to child. That difference makes women more valuable as hosts for the pathogens, the scientists claim. Previously, the study authors say, the variation of how a virus affected each sex had been attributed to differences in the immune systems.

"Survival of the fittest is relevant to all organisms, not just animals and humans. It's entirely probable that this sex-specific virulent behavior is happening to many other pathogens causing diseases. It's an excellent example of what evolutionary analysis can do for medicine," Úbeda said.

As New Scientist reports:

This evolutionary pressure, they argue, could explain a longstanding puzzle: why human T-cell lymphotropic virus type 1 (HTLV-1) progresses to leukemia much more commonly in Japanese men than Japanese women, but affects both sexes equally in the Caribbean. They argue that this discrepancy is because women breastfeed their babies more commonly and for longer in Japan — giving the virus more opportunity to enter another host.

Other infections that discriminate

In addition to HTLV-1, other infections cause more serious illnesses in men than in women. For example men are twice as likely to get tuberculosis, and men infected with the human papillomavirus are more likely to develop cancer.

A 2018 study found that even the flu virus discriminates by gender. Because women typically take on caregiver roles more than men, they are usually exposed to the flu virus more than their male counterparts. But when both men and women get the flu vaccine, its effectiveness for women was considerably higher (49 percent versus only 38 percent for men).

The next puzzle to solve, Jansen tells New Scientist, is how a virus knows whether the host is male or female. “There are all sorts of hormonal and other pathways that are slightly different between men and women,” he says. And if scientists can find the answer, it may lead to new medical treatments, he says. “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."

Editor's note: This story was originally published in December 2016 and has been updated with new information.

Angela Nelson ( @bostonangela ) is an exhausted mom of two young daughters and two old cats, and a Pulitzer Prize-winning digital editor with more than 15 years of experience delivering news and information to audiences worldwide.

Viruses may have evolved to go easier on women than men
New research shows that viral infections can evolve to affect men worse than women because the viruses consider women to be more valuable hosts.