Again, bacterial buildup (in the form of plaque) is a common cause of bad breath. But this isn't the only reason behind infection-related halitosis. Periodontal disease is an infection of the gums that can result in an unpleasant oral odor. Treating this condition is absolutely necessary — and not only to treat bad breath.
Untreated gum disease can cause pain, other dental issues (such as tooth loss), or systemic health conditions that affect other areas of the body, such as the heart.
Along with infections of the mouth, bad breath can come from nasal, sinus, or other upper respiratory infections. Tonsillitis and even strep throat can also result in bad breath. Brush as you might, overdoing it with oral care won't help these causes. You need to clear the infection before the smell will go away. Keep in mind, while you're caring for the infection, you still need to maintain proper brushing and flossing routines.