Medical research reports that women often exhibit stronger immune responses than men, while pathogens tend to be more virulent in men. Current explanations cannot account for this pattern, creating an obstacle for our understanding of infectious-disease outcomes and the incidence of autoimmune diseases. We offer an alternative explanation that relies on a fundamental difference between the sexes: maternity and the opportunities it creates for transmission of pathogens from mother to child (vertical transmission). Our explanation relies on a mathematical model of the co-evolution of host immunocompetence and pathogen virulence. Here, we show that when there is sufficient vertical transmission co-evolution leads women to defend strongly against temperate pathogens and men to defend weakly against aggressive pathogens, in keeping with medical observations. From a more applied perspective, we argue that limiting vertical transmission of infections would alleviate the disproportionate incidence of autoimmune diseases in women over evolutionary time.

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Much work has considered the evolution of pathogens, but little is known about how they respond to changes in host behaviour. We build a model of sublethal disease effects where hosts are able to choose to engage in prophylactic measures that reduce the likelihood of disease transmission. This choice is mediated by utility costs and benefits associated with prophylaxis, and the fraction of hosts engaged in prophylaxis is also affected by population dynamics. When prophylactic host behaviour occurs, we find that the level of pathogen host exploitation is reduced, by the action of selection, relative to the level that would otherwise be predicted in the absence of prophylaxis. Our work emphasizes the significance of the transmission-recovery trade-off faced by the pathogen and the ability of the pathogen to influence host prophylactic behaviour.

1Much work has considered the evolution of pathogens, but little is known about 2 how they respond to changes in host behaviour. We build a model where hosts are 3 able to choose to engage in prophylactic measures that reduce the likelihood of disease 4 transmission. This choice is mediated by costs and benefits associated with prophylaxis, 5 but the fraction of hosts engaged in prophylaxis is also affected by population dynamics. 6 We identify a critical cost threshold above which hosts do not engage in prophylaxis. 7Below the threshold, prophylactic host behaviour does occur and pathogen virulence, 8 measured by the extent to which it exploits its host, is reduced by the action of selection 9 relative to the level that would otherwise be predicted in the absence of prophylaxis. 10 Our work emphasizes the significance of the dual nature of the trade-off faced by the 11 pathogen between balancing transmission and recovery, and creating new infections in 12 hosts engaging or not engaging in prophylaxis.13