Kissing is considered a symbol of love and affection, but scientists claim it may have evolved for reasons less fascinating and more practical.
The scientists believe kissing is developed to pass on a dangerous bug called cytomegalovirus from man to woman.
The virus, which lurks in saliva, normally causes no problem but can prove dangerous during pregnancy, killing the unborn baby or causing birth defects, including problems ranging from deafness to cerebral palsy, claim the scientists.
"Female inoculation with a specific male's cytomegalovirus is most efficiently achieved through mouth-to-mouth contact and saliva exchange, particularly where the flow of saliva is from the male to the typically shorter female," said lead researcher Colin Hendrie.
However, kissing the same person for about six months helps the woman build up her immunity that minimises the risk of birth defects, Hendrie said in his writing in Medical Hypotheses journal.
As the relationships progresses and the kisses become more passionate, huge amount of virus passes to the woman, helping her build up her immunity.
"By the time she becomes pregnant, the odds of her unborn baby becoming infected are much lower," Hendrie added.
Earlier, scientists have claimed that kissing acts as a form of evolutionary quality control, with saliva holding clues to fertility, health and genes.