The gases that come out of the mouth that cause bad breath are volatile sulfur compounds, including hydrogen sulfide and methyl mercaptan. They can be generated by bacteria due to gum infection (gum disease) 

Morning breath, which can occur when you wake up, is caused by the breakdown of food particles overnight by bacteria.  The surface of the tongue has ridges that can trap food particles and bacteria that decompose and are not easily washed away by saliva because they reside inside the tongue ridges. 

Dietary causes of bad breath are the consumption of foods like onions, garlic, spices, pickles, chewing tobacco, and alcohol. Some foods such as garlic have a high amount of sulfur, and through continuous breakdown in the intestines can release gases for hours after consumption.

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