There are many causes for breath odor/bad breath/halitosis

If you don't brush and floss every day, food stays in your mouth and collects bacteria, which can cause bad breath. Food collected between the teeth, on the tongue and around the gums can also rot, leaving an unpleasant odor. Foods like garlic and onions contribute to breath odor, and once the food is absorbed into the bloodstream, it is transferred to the lungs where it's expelled. Brushing, flossing and mouthwash will only mask the odor temporarily; odors continue until the body eliminates the food.

Tobacco products are another cause of bad breath.