Ewww! Don't they know that kissing can transfer 250 colonies of bacteria from one person to another?

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When -- and more importantly, why -- did we decide that kissing was so great? Perhaps it's a relic of the innate reflex to suckle the breast (no, really). Or perhaps it's a learned behavior, one that possibly evolved from our ancient ancestors desire to identify the health and wellness of a potential mate through examination of the saliva [source: NPR]. Whether the behavior is innate or learned, it's not practiced across all cultures. While most of us kiss, nearly 10 percent of cultures around the world refrain from kissing, usually for reasons of cleanliness.

People have developed a particularly pleasant physical reaction to kissing and being kissed. The chemical oxytocin, which has been shown to bond humans to one another, is produced in the brain during a kiss. At this point, the reward center of the limbic system switches on and releases the chemical dopamine, which leads to the pleasurable feelings we experience during kissing. In other words, we get the same reaction from kissing that we do from chocolate and cocaine [source: Ahlstrom].

Kissing serves as an obvious emotional cue; it conveys affection, attraction and love. As thrilling as it is, however, it can also be dangerous (not to rain on anyone's parade or anything). The herpes simplex virus 1 (HSV1), which causes cold sores, can be transmitted from one person to another during a kiss. The same goes for syphilis. In fact, kissing is considered one way to transmit some sexually transmitted diseases. Of course, there's mononucleosis and glandular fever (the so-called kissing diseases), as well as colds, the flu and myriad other diseases that can also be transferred from a sick to a healthy person through kissing.

None of this should come as much of a surprise (at least not to the 10 percent of cultures that abstain from kissing). The average open-mouthed kiss can transfer around 250 colonies of bacteria [source: Harrison]. The mouth contains bacteria and saliva -- which, while relatively harmless, can actually hurt the thin skin of your lips. Find out on the next page how kissing can not only transmit disease, it can actually damage your lips.