Cytomegalovirus, which lives in human saliva, is generally innocuous, but when introduced during a pregnancy it can be extremely dangerous, killing unborn babies or causing birth defects like deafness or cerebral palsy. However, if cytomegalovirus is introduced to a woman in small doses before she conceives, she will build an immune resistance to it. Optimally, kissing the same male for six months prior to pregnancy gives the woman's immune system the practice it needs to render cytomegalovirus impotent.