According to The Telegraph,
this particular virus can cause blindness and other birth defects in infants, if the mother contracts it while she is pregnant.
According to Dr. Colin Hendrie of the University of Leeds, who published a report in the journal Medical Hypotheses, kissing allows for the virus to be spread, and therefore have an immunity built up to it, in preparation for the pregnancy. Dr. Hendrie says that kissing the same partner for at least six months is the best way to go about the immunity defense.
"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," Hendrie said.
Many theories have been launched as to why humans kiss in the first place. The most accepted theory is that kissing is a good way for the primal part of our beings to seek out a suitable mate LiveScience
writes that when our faces are together, our pheromones exchange biological information about whether two people will make strong, healthy offspring.