Be careful who you kiss. Scientists find a link between women's unexplained infertility and a little-known virus that is believed to be transmissible by kissing.

The findings could give renewed hope to couples and women who have been struggling with conception and do not know the cause of their infertility. It can also lead to new treatments that could target the virus.

Researchers from the Italy's University of Ferrara analyzed the uterus of 66 female participants, of which 36 have normal fertility while 30 have unexplained primary infertility. The team was led by Dario DiLuca, Roberta Rizzo and Roberto Marci.

They discovered that 43 percent of these women have been infected by one of the lesser-known herpes virus called HHV-6A. This particular virus has not been found in any of the uterus of the female participants whose fertility was deemed normal.

Among the women aged 15 to 44 years old, about 6 percent have infertility problems. Of these women, approximately 25 percent are unexplained. This pushes many women to undergo fertility treatments, which are often not just costly but also traumatic. One of these treatments is IVF, and many women are not sure if the treatments will ever work for them.

In the new study, the researchers wrote that the cytokine levels of the HHV-6A-infected women were abnormal. Cytokines are a big group of protein that signals the facilitation of cell interactions, which play vital roles in the supporting the fertilized eggs. Cytokines also play a role in fetal development.

The HHV-6A-infected women also had higher levels of estradiol, a hormone that varies depending on the woman's menstrual cycle. Estradiol could trigger an HHV-6A infection. The virus multiplies in the salivary glands, however, it is not usually detectable in the saliva or blood. Therefore, the state of its true frequency is still unknown.

"This is a surprising discovery. If confirmed, the finding has the potential to improve the outcome for a large subset of infertile women," said Anthony Komaroff, a Harvard Medical School professor who studied the virus.

The researchers said that further research is required to confirm the preliminary findings. Additional studies will also be needed to see if an antiviral therapy can help the women who are infected by the HHV-6A virus.

To date, the scientific and medical communities have little data on HHV-6A. The virus was first discovered in 1986. It is one of the eight herpes viruses that infect humans.

The research was published in the PLOS One journal on July 1.