The cause of bad breath dpends on what's causing the odor. Palque is a common cause. Gastro-intestinal tract anf lung disorders can also cause bad breath. The strong odors of garlic, onions, and alcohol are carried through the blood stream and exhaled through the lungs.
Tobacco is one big loser.
Other health problems like sinus infection, diabetes and severe gastric ulcers can also cause bad breath. Meticulous oral hygiene is the very basic tenet for a fresher breath. Brushing your teeth at least twice a day prevents plaque formation. However, food can be trapped between teeth and at gum lines. A regular flossing habit is needed to remove this.