Between three and four hundred types of microbes typically call the human mouth home. Most are harmless, but a few produce 'volatile sulfur compound.' These compounds, such as hydrogen sulfide, which smell like rotting eggs, give your breath its characteristic bad odor.

VCS-producing bacteria are generally present in inoffensive quantities. However, as oxygen levels in the areas of the mouth are reduced, behind a layer of plaque or food, for example, these anaerobic bacteria thrive. As VSC-levels increase, the pH of the mouth is reduced, which creates the sour, metallic taste experience by halitosis sufferers.

So how can you treat bad breath? By brushing and flossing, of course. Oh, and avoid onions and spicy foods. They are very high in sulfur content.