Decayed teeth can contribute to bad breath. Dental cavities are actually bacterial infections. If you have a cavity or loose filling, there can be significant odor. These areas are impossible to clean with the toothbrush and floss, because they are like caves teeming with bugs. Many times people will identify a cavity or loose filling by having a bad taste in their mouths. The worst case scenario is a dental abscess. With regular dental exams, including dental radiographs, you can identify problem areas and have them treated.