How many germs can you get from kissing?
It doesn't matter how many times you got the cootie shot on the
playground; when you kiss another person, you're going
mouth-to-mouth with their germs. When you kiss you swap germs by
swallowing spit.
Also most importantly, when a man kisses a female partner, he
passes a small amount of his cytomegalovirus to her. If the
cytomegalovirus is introduced into a woman's system during
pregnancy, it can damage or even potentially kill the fetus. But,
if a woman kisses the same partner repeatedly, she eventually
develops an immunity to his particular cytomegalovirus, decreasing
the chances of infection during pregnancy.