Gum Disease #1. The #1 reason for bad breath is gum disease. Volatile sulfur compound (vsc) are released which causes the rotten egg smell. There are mouth rinses that will neutralize vscs, but the source, which is most likely infected gums, needs to be treated. You will need to see your dentist to treat any gum infection and your physician to rule out a sinus infection or other possible source of your problem.
Bad. Bad breath (halitosis) can be the result of many medical issues or simply the result of your diet. For example, many people are unaware that their diet has garlic (in prepared meals, for example), which is metabolized in such a way that sulfur is expelled (from your lungs, not your mouth), and no amount of teeth cleaning or gargle will cure it. It takes a day or so to work its way out. Sinus infections, tooth infections and allergic rhinitis can cause bad breath as well. Some people develop white deposits in their tonsils called "tonsoliths" that may suggest an infection and that smell bad also. See you doctor and see which if any of these may be the culprit as all of them have simple remedies. Good luck!