It predicts that the adventurous lifestyle of the male means that they are more exposed to disease but paradoxically this reduces their immunity.  The reason is that they invest more energy in maintaining the ability to reproduce while ill and also take the view they will be reinfected quickly so do not need to have such a strong immune system.  Dr Olivier Restif, of the University of Cambridge, said: "In many cases, males tend to be more prone to get infected or less able to clear infection.  "Proposed mechanisms include interference between male hormones and immunity, as well as risk-taking behaviour.  Men compete more with each other than females do, because of the male strategy to "live hard, die young' which means they may invest less in immunity.  Dr Restif and colleague Dr William Amos assessed how the evolution of male and female immune defences is affected by differences between the sexes.  Dr Restif said: "An increase in male susceptibility or exposure to infection favours the spread of the pathogen in the whole population and therefore tends to select for higher resistance or tolerance in both sexes if the cost of immunity is essential.  "But above a certain level of exposure, the benefit of rapid recovery in males decreases owing to constant reinfection.  "This selects for lower resistance in males, ultimately leading to the counterintuitive situation where males with higher susceptibility or exposure to infection than females evolve lower immunocompetence."  The finding published in Proceedings of the Royal Society B: Biological Sciences suggests that 'man flu' is not a myth.  Across a range of animal species, males tend to be the "weaker sex" in relation to immune defences, usually because of hormonal differences.  Dr Restif said: "Our results show that, under a range of genetic and ecological constraints, males and females can evolve different levels of immune defences, sometimes at odds with intuitive expectations."  He said that maintaining the ability to mate was more important to men than getting better, yet for women it was the other way around.  "Under this scenario, the model predicts that overall, females should try to clear infection rapidly, regardless of the relative risk of catching infection," he said.  "In contrast, males are selected to decrease their immune defences and remain sexually active during infection if their exposure to infection is high."  "On the other hand, even if males have a strong immune system that clears infection, they will become reinfected rapidly, so the effective benefit of immunity is low."
Man flu is not a myth, claim scientists, who found men suffer more because   they invest in their spirit of adventure at the expense of their immune   system.