Bad breath can come from several sources, but one major culprit is food. We all know the usual food suspects that cause bad breath, but why? The answer is that the food you eat gets absorbed into your bloodstream where it transfers to your lungs. The fact of the matter is that you can only completely eliminate this type of bad breath once your body disposes of the food that caused it. Another culprit of halitosis is bacteria. When food stays in your mouth it can become a breeding ground of bacteria that omits a fowl odor. Dry mouth can also cause bad breath since saliva that is needed to clean your mouth becomes sparse and thus bacteria again forms from existing food particles.
One of the warning signs of periodontal (gum) disease is persistent bad breath or a bad taste in the mouth. Periodontal disease is caused by plaque, the sticky, colorless film of bacteria that constantly forms on teeth. The bacteria create toxins that irritate the gums. In the advanced stage of the disease, the gums, bone and other structures that support the teeth become damaged. With regular dental checkups, your dentist can detect and treat periodontal disease early. (Source ADA)
It is further good to know that bad breath can be symptomatic of a deeper underlying medical issue. Infections and other maladies can cause a fowl odor to omit from your mouth that has less to do with oral health, and much more to do with your overall health. In situations like these your dentist will likely refer you to your primary care physician.