It is caused by anaerobic bacteria producing the following gases:
§Cadaverine : as from corps
§Hydrogen sulfide : as from rotten eggs
§Putrescine : as from decaying meat
§Methyl mercaptan
§Skatole : as from faecal matter
§Isovaleric Acid : as from sweaty feet
§Volatile Sulphur Compounds.
We all have them!!!
But if they are produced in excess,
you have Halitosis.