There is such a thing as man flu, with men less resistant to pneumonia and other respiratory illnesses, according to new research.

Scientists from Harvard University have discovered that the female sex hormone oestrogen fortifies the immune system, and men are suffering for its absence.

In the study, published in Life Sciences medical journal, a simple dose of oestrogen was capable of curing both male and female mice of bacterial pneumonia.

Professor Lester Kobzik at the Harvard University School of Public Health said female mice demonstrated increased resistance to infection, linked to the enzyme nitric oxide synthase 3 (NOS3) activated by the release of oestrogen.

After introducing the bacteria streptococcus pneumoniae into the lungs of mice, the research team found oestrogen treatment to accelerate recovery.

In another round of tests, scientists removed the gene response for the production of NOS3, an enzyme activated by oestrogen, and found the female mice had lost their resistance to infection.

It is hoped that this discovery could be used to improve resistance to common and serious lung infections and prevent flu from developing into pneumonia.

Professor Kobzik said: "Ultimately, this work could be especially useful in reducing risk of secondary bacterial pneumonias during seasonal or pandemic influenza.