Bad breath or halitosis in dentistry can be the outcome of a variety of mouth conditions like mouth ulcers, decaying teeth, or infected gums. Halitosis can be usually treated by treating the underlying cause and by maintaining proper oral hygiene. 

Other than oral conditions; halitosis can also be a byproduct of GIT (Gastro Intestinal or stomach) related issues. In this case; consulting with a medical practitioner is required. But consulting with a dentist is also recommended to rule out any possible dental issue(s).