While women have for centuries been labelled as the weaker sex, new research suggests the truth is quite the opposite.
According to a study, the fairer sex is genetically programmed to better resist infections and cancer, and also have a back-up system for fighting disease.
The discovery sheds light on why members of the so-called stronger sex succumb to "man-flu".
Their immune systems are no match for those of wives and girlfriends because of the female X-chromosome, scientists believe.
The reason why women are more robust appears to be microRNAs - short strands of RNA encoded on the chromosome.
RNA is a genetic cousin of DNA and can have important biological effects.
The microRNAs have the effect of "silencing" immunity genes on a man's X-chromosome, according to the new research.
This leaves men at a disadvantage since they only have one X chromosome. Women have two, so that even when immunity genes are silenced on one the other can compensate, according to research published today in the journal BioEssays.
Study leader Dr Claude Libert, from Ghent University in Belgium, said: "Statistics show that in humans, as with other mammals, females live longer than males and are more able to fight off shock episodes from sepsis, infection or trauma. We believe this is due to the X chromosome which in humans contains 10 per cent of all microRNAs detected so far in the genome (genetic code)."
Several X-chromosome located microRNAs were thought to have "important functions in immunity and cancer", said Dr Libert.
From a biolgoical point of view, the difference has probably evolved because women are more likely to ensure the survival of the species. They need to be able to resist infection when pregnant and when nuturing a child.