Bad breath is caused by a variety of factors. In most cases, it is caused by food remaining in the mouth—on the teeth, tongue, gums, and other structures—and collecting bacteria. Dead and dying bacterial cells release a sulfur compound that gives your breath an unpleasant odor.

Certain foods, such as garlic and onions, contribute to breath odor. Once the food is absorbed into the bloodstream, it is transferred to the lungs, where it is exhaled. Brushing, flossing and mouthwash only mask the odor. Dieters sometimes develop unpleasant breath from fasting. Periodontal (gum) disease often causes persistent bad breath or a bad taste in the mouth, and persistent bad breath may mean a sign that you have gum disease. Gum disease is caused by plaque - the sticky, often colorless, film of bacteria that constantly forms on teeth.

Dry mouth, or xerostomia, may also cause bad breath due to decreased salivary flow. Saliva cleans your mouth and removes particles that may cause odor. Tobacco products cause bad breath and stained teeth, they reduce your ability to taste foods, and they irritate your gum tissues. Bad breath may also be a sign that you have a serious health problem, such as a respiratory tract infection, chronic sinusitis, postnasal drip, chronic bronchitis, diabetes, gastrointestinal disturbance, or liver or kidney ailment.