I have a personal theory that explains man-flu. This theory has not been tested, but I feel that there is a perfectly reasonable excuse for the fact that men suffer more than women. This is based on medical facts that are well known, and easily proven.
FACT: During the course of a month, a woman's body temperature WILL fluctuate, in fact, many women monitor these changes to predict their most fertile days, if they are trying to conceive.
FACT: Men are lucky in that during a normal month, their temperature will remain constant.
THEORY: Men are more susceptible to temperature changes in their bodies than women, and will therefore feel the effects more.
(PROOF of THEORY: I have been explaining this to Mrs B for years...and she even believes it too!)