Bad breath (a.k.a. oral malodor/fetor oris/halitosis) is a very common but insidious human predicament, in that it is rarely experienced by the offender. Since it is so personal, the one having it may never get a clue from those affected by it.
Studies indicate that this condition arises directly from either exhaled digestive gases, various conditions within the mouth, or a combination of both. Dental plaque bacteria that reside between the teeth and gums, tongue, and cheeks can absorb certain foods that have a high content of volatile sulfur compounds (VSC's). This alone can be offensive, especially the morning after a meal high in VSC's. Aside from the foods that have a high content of VSC's, the plaque by itself that causes gum disease (periodontal disease or pyorrhea), is definitely the most common cause of bad breath. Add to this, high VSC foods such as garlic, etc., and you have a walking halitosis factory - an offender usually not "in the know".