This condition is technically called as Halitosis. The most common reasons for bad breath are decayed teeth and gum infections. Other possible reasons might be ENT problems, lung infections, gastric problems, kidney related diseases, Diabetes, smoking, alcohol, certain food materials and many more. 

The most effective way to control bad breath is to maintain good oral hygiene i.e brushing twice daily, getting the teeth cleaned regularly and getting any tooth cavities filled. Lifestyle changes such as taking a balanced diet especially the one which is rich in fibers and consumption of good quantity of water so that the food debris gets washed away, and keeping the mouth wet are the most efficient home care measures to avoid bad breath.