What happens when we kiss?
A kiss is a compelling combination of anatomy, sensation, and emotion. It begins with a tilt of the head, usually to the right. Facial muscles, with lovely names like orbicularis oris and zygomaticus, coordinate a sequence of intricate motions between lips, cheeks and, if you choose, the jaw and the tongue. A rush of hormones and sensory messages flood the body and brain, and the heartbeat flutters faster.

Why does a kiss feel so good?
With all of the body parts involved, our lips are at the heart of the kissing pleasure principle. Gray's Anatomy (the essential anatomy text, not the nighttime soap opera) calls them "two fleshy folds," but they're more than that. They're flexible, receptive, and oh-so-sensitive.

The very thin layer of skin that forms the outside of the lips is continuous with the inside lining of the mouth, containing mucous membranes, tiny muscles, and the many nerves that can help to make a kiss feel sublime. Kissing also releases endorphins, the same hormone that triggers a "runner's high" and acts as the body's natural painkiller.

Why do we kiss?
When we pucker up, we are part of the estimated 90% of humankind that kisses in some way, shape, or form. Most of us are doing it, but why we're all smooching is still not too clear.

Some theorize that the origins of modern kissing are maternal. It's thought that human mothers may have once fed their babies like birds do, by chewing food first and then passing it from their mouth into the mouth of their infant. This mouth-to-mouth contact may have evolved to be a comfort when food was scarce and eventually to just plain comforting. That characteristic right-sided head tilt before a kiss could even be linked to the way that most infants turn to the right to nurse while breast-feeding.

To try to get at the reasons why we kiss, research published in the journal Evolutionary Psychology examined the kissing attitudes and behaviours of over 1,000 college students. What researchers discovered was that men and women tend to use kisses for different purposes. Women more often kissed as a way to check in on the status of a relationship and to size up the viability of a mate, while men saw kisses as more of a means to an end - that end being arousal and sexual intercourse.

What are the consequences of a kiss?
Kisses can have contagious and occasionally deadly consequences. Mononucleosis, the so-called "kissing disease," can be spread by swapping spit with a kissing partner. Extreme fatigue, sore throat, swollen lymph nodes, and fever can linger for weeks longer than the brief bliss of that kiss.

Some sexually transmitted infections may be spread by kissing, like the mouth sores that are caused by herpes. HIV cannot be spread through casual contact, like dry kissing, but the risk may increase if mouth sores were present and if kisses were very wet.

Parents and caregivers who kiss their babies and children may pass along bacteria that could cause tooth decay or the virus that can lead to respiratory syncytial virus (RSV), a common infection that can turn serious in some infants.

There have been a few rare cases of what some would call the "the kiss of death." A young teenager with a serious peanut allergy died after kissing her boyfriend, who had earlier in the day eaten something containing nuts. In another case, a woman's shellfish allergy and her mate's meal of shrimp combined to create an almost lethal kiss.

Another rare but notable case of kiss-with-caution is "the kiss of deaf," in which an especially powerful kiss can actually affect a person's eardrum and lead to temporary hearing loss. In these cases, it's not a loud kiss that causes the ear damage - it's the suction.

What are the benefits of a kiss?
Don't be frightened away by stories of fatal kisses. Most kisses are safe, and they usually range from mildly sweet and pleasurable to downright knee-buckling and earth-shattering. A kiss can relax us or arouse us and make us feel more connected. A kiss can also tell us about our relationships, sometimes saying more than words can convey.

Kissing may be just the thing for your allergies, too. Japanese researchers found that kissing may cause the body to slow down its production of the histamines that trigger allergy symptoms.

Amy Toffelmire