[Phenomenon]

Men may have a weaker immune system and could be more vulnerable to so-called 'man flu', scientists propose. Evolutionary factors and hormonal differences may make males more susceptible to infection than females, says a Cambridge University team. Their theory, outlined in a scientific journal, suggests there is a trade-off between a strong immune system and reproductive success. But a leading flu expert says there is no difference in men's immunity. Previous experiments have found differences in the ability of females and males to deal with infection.

Across a range of animal species, males tend to be the 'weaker sex' in terms of immune defences, says the Cambridge team. This is usually explained by the difference in hormones. High testosterone levels, they say, lead to more coughs and colds. The study, published in Proceedings of the Royal Society B journal, investigated the reason for these differences. Dr Olivier Restif, one of the authors, said: "If you assume that males are more exposed to infection, then can natural selection influence that process?" To find out, scientists developed a mathematical model that highlighted the role of other factors, like ecology and epidemiology, in shaping the immune systems of men and women. "If males are more exposed to infection than females (for behavioural reasons for example), it is possible to see them evolve lower immuno-competence than females," the authors wrote.

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