Basic dental hygiene removes particles of trapped food that can create an unpleasant mouth odor as the food decays. But a variety of other things can contribute to bad breath, such as:
• Dry mouth. Without the cleansing power of saliva, dead cells build up and decay in your mouth. This is the source of the phenomenon called morning breath, and it’s particularly bad if you sleep with your mouth dangling open.
• Digested food. Certain foods, like garlic, have volatile oils that can stink up your airways. There’s no way to rid yourself of these odors, because the odor begins after you absorb and process the food—and it actually seeps out of your lungs. Fortunately, the scent should die down in 24 hours. In the meantime, you can try to mask it by chewing on a clove, some mint, or a sprig of parsely (and hope that this combination doesn’t create a still more objectionable smell).
• Dental problems. When bacteria works itself into places it shouldn’t be—such as the pockets between your teeth and gums—it’s impossible to remove on your own. The problem usually begins with poor dental hygiene, and you can only fix it with a trip to the dentist.
• Diseases. Certain medical conditions can produce strange or offensive smells. For example, untreated diabetes can cause a fruity smell. Kidney failure can cause an ammonia-like smell. If you suddenly develop a new and unpleasant mouth odor, check it out with your doctor.
Source of Information : Oreilly - Your Body Missing Manual