In infants, bad breath usually accompanies the advent of teeth. The gums become a little irritated and can smell foul when the new choppers pierce through. In older kids, bad breath could result from the appearance of back teeth as well as from mouth breathing at night, which dries the mouth and allows bacteria to flourish. Enlarged adenoids are the main cause of mouth breathing [See: Adenoids]. Finally, a persistent foul odor in Jimmy’s mouth could indicate that he has inserted something in his nose, which should be removed by your doctor.