Seventy-one percent of teens say it is an instant turn-off. Eighty-five percent say it's the most important thing to avoid when meeting someone for the first time. Health columnists say it is the medical problem they receive the most inquiries about. Although it can be avoided, the most worrisome aspect about it is that you might not even know if you've got it. It is embarrassing, demoralizing, all-too-common bad breath.