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1 Persistent bad breath or a bad taste in the mouth can also be one of the warning signs of gum disease; which is caused by plaque.  Dry mouth. This occurs when the flow of saliva decreases and can be caused by various medications, salivary gland problems or continuously breathing through the mouth.
In addition, bad breath can be caused by the following: Poor dental hygiene â Infrequent or improper brushing and flossing, allows bits of food that are stuck between the teeth to decay inside the mouth. Poor oral hygiene eventually will lead to periodontal (gum) disease, which also can cause bad breath. Infections in the mouth â These can be caused by either a cavity in a tooth or by periodontal (gum) disease.