Halitosis (bad breath) is mostly caused by sulphur-producing bacteria that normally live on the surface of the tongue and in the throat. Sometimes, these bacteria start to break down proteins at a very high rate and odorous volatile sulphur compounds (VSC) are released from the back of the tongue and throat.
https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/halitosis-or-bad-breath

