Bad breath- medical term: Halitosis
The cause of bad breath dpends on what's causing the odor. About 80-90% of cases is attributed with what's  inside the mouth. Palque is a common cause. Other causes are gum diseases(e.g. gingivitis) and food impacted inside decayed tooth.
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.
Bacteria can lodge in your tongue. So be sure to brush it after your teeth.
Ever wonder of the unpleasant morning breath? Salivary flow is decreased during night time and during sleep. Saliva has a natural antibacterial action, and when you have least of it, bacterial proliferation is increased. Chewing on sugarless gum or candy can step up salivary flow.
When brushing every after meal is a burden, rinse always with plenty of water. This aids in reducing food particles that are impacted in the teeth. 
Munching on crunchy vegetables and fruits produces a polishing action teeth and a massaging action on your gums. Isn't a vegetable salad delicious aside from being nutritious?
Common mouthwashes may cover bad breath, but only for a short period of time. However, there are new brands of mouthwashes that aren't just breath freshener, but are antiseptic as well.