What are the most common causes of bad breath?

1) Dry mouth (which can be caused by many common medications, mouth breathing, or lack of saliva flow)
2) Gum disease - this normally does not hurt or have obvious symptoms, but is extremely common and can cause unnecessary tooth loss
3) Cavities or ill-fitting dental restorations such as fillings and crowns
4) Poorly fitting dentures
5) Improper cleaning of the tongue, teeth and gums