Halitosis is mostly caused by anaerobic bacteria, states TheraBreath.com. Anaerobic bacteria don't require oxygen to survive and are only found in areas in the body that are exposed to the least air such as the intestines, mouth, nose and throat.
Conditions that contribute to the growth of anaerobic bacteria are poor oral hygiene, dry mouth, sinus infections and oral infections. Not brushing and having poor oral hygiene leads to halitosis because bacteria builds up over time in the mouth, notes BreathMD.com. A dry mouth can cause a buildup of anaerobic bacteria because saliva contains oxygen, which helps kill bacteria. Sinus infections and oral infections are also other causes of anaerobic bacteria build up because the conditions increase the amount of bacteria present in the mouth.