It has long been known that the human mouth is filled with bacteria. There are an estimated 100 to 200 species that normally dwell there! That means hundreds of thousands of those microscopic creatures. Most of them are either harmless or even beneficial to the person who hosts these bugs, but transferring them to someone else can sometimes be quite harmful. As a plastic surgeon I was taught that a facial wound from a dog bite can be cleansed and sutured if treated soon after the injury; but a human bite is much worse and will almost always become infected if treated similarly. So a surgeon should never close a human bite immediately, but rather disinfect it, leave it open until it heals and then revise the scar later on.
But wait. A recent study showed that just a 10 second kiss can transfer about 80 million bacteria between osculaters, so think about that on your next date and start smooching at your own risk. Mouthwash anyone?
[On the other hand, some believe The 10-Second Kiss can save a marriage]