Bad breath is breath that has an unpleasant odor. It's also known as halitosis. This odor can occur from time to time, or it can be long lasting, depending on the cause. Millions of bacteria live in the mouth, particularly on the back of the tongue. In many people, they are the primary causes of bad breath. The mouth's warm, moist conditions are ideal for the growth of these bacteria. Most bad breath is caused by something in the mouth

Infections in the mouth:

These can be caused by either a cavity in a tooth or by periodontal (gum) disease.

Respiratory tract infections:

Throat, sinus or lung infections

External source:

Garlic, onions, coffee, cigarette smoking, chewing tobacco

Dry mouth:

This can be caused by salivary gland problems, medicines or "mouth breathing." A large number of prescriptions and over the counter medicines cause dry mouth

Illnesses:

Diabetes, liver disease, lung disease, sinus disease etc..