Also known as halitosis, is breath that has an unpleasant odor. This odor can strike periodically or be persistent, depending on the cause. In many people, the millions of bacteria that live in the mouth (particularly on the back of the tongue) are the primary cause of bad breath. The mouth's warm, moist conditions make an ideal environment for these bacteria to grow. Most bad breath is caused by something in the mouth.
Some types of bad breath, such as "morning mouth" are considered to be fairly normal, and they usually aren't health concerns. The "morning mouth" type of bad breath occurs because the saliva that regularly washes away decaying food and odors during the daytime diminishes at night while you sleep. Your mouth becomes dry and dead cells adhere to your tongue and to the inside of your cheeks. Bacteria use these cells for food and expel compounds that have a foul odor.
Dr. James L. Johnson