The idea that men suffer more from some infectious diseases is not new. So-called "man flu" is prevalent in pop culture, and the explanation has often pointed toward the antiviral potential of estrogen and women's superior immune systems. But some scientists feel that there is more to learn about this phenomenon. For instance, such diseases don't appear to show a sex bias "immediately after puberty—when sex-hormones are first produced—but almost a decade later," researchers explain.
They believe that there is an evolutionary explanation for the sex bias of some viruses. Their study, "The evolution of sex-specific virulence in infectious diseases," uses mathematical models to explore this theory. According to the findings, some viral illnesses, such as HPV, evolve to be less harsh on female hosts because the bodies of women present greater opportunity to the virus to subsequently infect other hosts, such as offspring.
There are two forms of viral transmission discussed in this study. One is called horizontal transmission, which refers to the passage of an infection between people within a population. The other is called vertical transmission, which is about giving babies diseases through pregnancy, birth, and breastfeeding.
"Because women can transmit pathogens during pregnancy, birth, or breast-feeding, pathogens adapt, evolving lower virulence in women," the study reads. The researchers refer to several sad facts: The bacteria that causes tuberculosis apparently kills men nearly twice as much as women and, in men, HPV is "five times more likely" to develop into the potentially fatal tonsil cancer than in women.
Ultimately, they use their model to interpret data from Japan, where the virus that causes adult T-cell leukemia is more likely to develop into that lethal disease if its host is male. This sex difference, while found in Japan, is not found everywhere, such as the Caribbean. The researchers' theory posits that infectious diseases go through a process of natural selection based on their ability to spread in different hosts. "We argue that breastfeeding, being more prolonged in Japan than in the Caribbean, may have driven the difference in virulence between the two populations," the study reads.
It is possible that other factors could cause the differences that are seen between Japanese and Caribbean populations: "It is well-known that there are many risk factors for different diseases other than sex and gender," says study author Vincent Jansen, a professor of Mathematical Biology at Royal Holloway University in London.
He explains that the mathematical model created in his study—which predicts that pathogens become less virulent in hosts that provide an increased opportunity for vertical transmission—aligns with the data about leukemia among men and women in both Japan and the Caribbean. This underscores the reason why he and his fellow researchers felt it was important to do this study at all: "If the differences are caused by differences in hormones between men and women, or by differences in the immune system, why then do we see differences between different geographical regions?"
"If a virus is transmitted from mother to child, women are more valuable to the virus than men are," Jansen says. It isn't clear how infectious diseases can even tell whether they're inside of a male or female host. According to Jansen, scientists don't know why that is right now, but the findings in this study "indicate that there is some sort of cue that the pathogen uses." He hopes that cue will be identified, because then it may "provide a starting point for trying to trick pathogens to become less virulent."