Generally there are two causes for offensive breath.

First, systemic diseases such as respiratory diseases, gastrointestinal diseases, diabetes, urine poisoning (uremia) etc. can produce bad breath.

Second, oral diseases cause bad breath. So do cigarettes and alcohol.

Most of all, the most common oral disease is tooth decay.
To sweeten the air, the decayed area is removed and in-layed with filling materials such as silver or gold and canal treatment (endodontic treatment) should be applied.
Besides this gum inflammation can also be a culprit of halitosis. Calculus {calcified plaque or weak sediments formed on the surface of the tooth or gums or other oral structures} gluing to gums results in gum inflammation which is called periodontal disease. As periodontal disease worsens, the alveolar bones that support the teeth crumble and in the worst case, those teeth should be extracted. Scaling is a treatment for removing calculus or the major cause of the periodontal disease, sediments, and discoloration.
Using mouthwash alone without any other dental treatment rarely works.
Consequently, identifying the exact cause of bad-breath odor and receiving the fundamental treatment should be in order. When there is no unusual symptom, proper brushing three times a day more or less helps prevent halitosis.