Man flu may not be a myth after all, as scientist have found that men with high levels of testosterone have a hidden flaw - weak immune systems.
The discovery could explain why men are more susceptible than women to a whole range of bacterial, viral, fungal and parasitic infections, researchers said.
It may also be the reason why men's immune systems respond less strongly to vaccinations against influenza, yellow fever, measles and hepatitis, along with many other infectious diseases.
Those who take testosterone supplements in the quest to gain muscle meanwhile, could be making themselves more susceptible to illness.
“This is the first study to show an explicit correlation between testosterone levels, gene expression and immune responsiveness in humans,” said US lead scientist Professor Mark Davis, from Stanford University.
“It could be food for thought to all the testosterone-supplement takers out there.”
The researchers studied how the immune systems of 34 men and 53 women were stimulated by the flu vaccine.
The jab generated a bigger boost in protective antibodies in women, with further analysis revealing activity that, in high testosterone men, was associated with a weakened antibody response. Men with low testosterone were not affected the same way.
Testosterone's anti-inflammatory properties may explain why it can weaken the immune system, said scientists writing in the journal Proceedings of the National Academy of Sciences.
Prof Davies said the reason why testosterone weakens the immune system yet boosts muscle power and aggression, may be linked to the man's evolutionary role.
Men are more likely than women to suffer injuries from competitive encounters, as well as their traditional roles of hunting, defence and potentially dangerous physical work, Prof Davies said. The dampening down the immune system makes male less susceptible to a potentially fatal over-reaction to infections, especially those from wounds.
“Ask yourself which sex is more likely to clash violently with, and do grievous bodily harm to, others of their own sex,” Prof Davis added.