Poor oral hygiene, gum disease, tooth decay, or mouth infections can also be causes of halitosis. Infections in the lungs, sinuses, or airways can also cause bad breath due to the presence of nasal secretions that may drain into the mouth. Smoking can also precipitate other bad-breath causes such as gum disease or oral cancers. 2  Poor dental hygiene: When a person does not brush or floss regularly, food particles remaining in the mouth can rot and cause bad odors.