The evolution of sex-specific virulence in infectious diseases

The fatality rates of infectious diseases are higher in men than women. Although this difference has been attributed to a stronger immune response in women in men, we show that differences in the transmission routes that the sexes provide can result in evolution favouring pathogens with sex-specific virulence. Because women can transmit pathogens during pregnancy, birth or breast-feeding and men cannot, pathogens adapt evolving lower virulence in women. This result can resolve the long-standing puzzle on progression from Human T-cell Lymphotropic Virus Type 1 (HTLV-1) infection to lethal Adult T-cell Leukemia (ATL); a progression that is more likely in Japanese men than women, while it is equally likely in Caribbean women and men. We argue that breast-feeding, being more prolonged in Japan than in the Caribbean, may drive the difference in virulence between the two HTLV-1 endemic populations. Our finding signifies the importance of investigating the differences in genetic expression profile of pathogens in males and females.