We know that proteins contain amino acids, which in turn contain sulfur compounds (see text above). When a person has a post nasal drip, mucus drains and coats the back of the tongue and throat, exactly where bacteria live (see diagram above). Since mucus is made up of interlinked strands of protein, the bacteria have a field day, breaking down these proteins into odorous and sour tasting sulfur type compounds. Strong Antihistamines don't help, because the drying effect of the antihistamines also creates a problem. What about the sinuses? First of all, these anaerobic bacteria cannot live in the sinuses. When someone has a sinus infection, one of the common symptoms are intense sinus headaches, caused by the pressure from the infection in the sinus. If you don't have these powerful headaches, you probably don't have a sinus problem. The fact is that once someone has an elevated amount of these anaerobic bacteria, they will create the problem from any protein source, including mucous, phlegm, etc, which drain down the back of your throat into the area where these bacteria live.