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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.
Some of the most common causes of bad breath are obvious, like you know it's time to brush when you've had some particularly garlicky food or a good curry and you practically breathe fire first thing in the morning. Many cases of bad breath are caused by untreated tooth decay, gum disease and mouth infections.