The next time you kiss someone, think about this: in your mouth, and in the mouth of every adult, live more than 400 different species of microorganisms, mostly bacteria. Billions and billions of them grow in layers, crowded together and wrapped cozily around each other, on every slimy surface, dark nook, and inviting cranny. It's enough to make a body want to keep lips permanently pursed.

With an average temperature of about 95 degrees, a saliva-induced humidity of 100 percent, and regular stoking with sugar and other simple carbohydrates-manna from bacterial heaven-the mouth provides a home for such a diversity of species that it could be called the tropical rainforest of the body. "In one mouth, the number of bacteria can easily exceed the number of people who live on earth," says Sigmund Socransky, a dental researcher at the Forsyth Dental Center in Boston, Mass. "In a clean mouth, 1,000 to 100,000 bacteria live on each tooth surface. A person who doesn't have a terribly clean mouth can have 100 million to 1 billion bacteria growing on each tooth."