I'm wondering what is the main cause of permanent bad breath?

Gum disease: The main cause of bad breath is gum disease. See a dentist for an evaluation.

Dr Krauser agreed

See a dentist: Halitosis, bad breath is caused by poor dental health, tooth decay and may be a sign of other health problems. Also the type of food you eat or other factors like smoking can effect your breath. Rec.: see a dentist to determine the cause of the problem and proper treatment.

Dr Munshower and another doctor agreed

Bacteria: Main cause oh halitosis (bad breath) is sulphur producing bacteria. Bacteria may be involved with infections (teeth, gums, tonsils/adenoids) or can collect between teeth or on tongue. May also be due to foods, intestinal problems, medications, and/or systemic illness. See both your General Dentist and your General Physician to determine the cause(s) of your problem.