A team at the University of Ferrara in Italy examined the uteruses of women with unexplained primary infertility - the inability ever to bear a child - and found that 43 per cent of them were infected by HHV-6A, one of the human herpes viruses.

The virus was not found in any of the women whose fertility was normal. Infertility affects approximately 6 per cent of women aged from 15 to 44.

About a quarter of these cases are unexplained, prompting many women to opt for expensive and traumatic fertility treatments, such as IVF, with no way of knowing if they are likely to work.

Published in the journal PLoS One, the study describes how all the women infected with HHV-6A were found to have abnormal levels of cytokines, which are signalling proteins that facilitate interactions between cells that play an important role in supporting fertilised eggs and foetal development.