Bad breath is caused by odor-producing bacteria that grow in the mouth. When you don't brush and floss regularly, bacteria accumulate on the bits of food left in your mouth and between your teeth. If you're concerned about bad breath, make sure you're taking care of your teeth and mouth properly. Some sugar-free gums and mints can temporarily mask odors, too. If you brush and floss properly and visit your dentist for regular cleanings, but your bad breath persists, you may have a medical problem like sinusitis or gum disease.