The glans of the penis is covered by 17,000,000 bacteria . The vaginal cavity is lined with a layer of 10,000,000 microbes per square centimeter. In a standard sexual act, then, a multibillion-dollar bacterial exchange occurs, and 250 different viruses are transmitted.
A kiss, in that sense, is not much more chaste. Fortunately, all those foreign bacteria that pass into your mouth barely live a day .
According to a study , a single passionate kiss promotes the transfer of up to 1 billion bacteria from one mouth to another , along with about 0.7 milligrams of protein, 0.45 milligrams of salt, 0.7 micrograms of fat, and 0.2 micrograms of "mixed organic compounds", which in Roman paladin means "food scraps."
However, microbes tend to jealously defend their territory, which is why, after the kiss, as Bill Bryson explains in The Human Body :
The host microorganisms in the two people involved initiate a kind of gigantic cleaning process, and in a matter of a day or so the microbial profile of both parties will have more or less fully restored to what they had before the tongues were intertwined. Every now and then some pathogens creep in, and that’s when we catch a cold or herpes, but that’s the exception rather than the rule.
During the Middle Ages it was decreed that kissing, like love play or foreplay in general, was reprehensible. And Sigmund Freud classified the kiss as "perversion" in his Three Essays on the Theory of Sexuality. And that they did not know about this multimillion-dollar crossbreeding of bacteria.