Bad Breath Causes

Hygiene, personal habits, pre-existing medical conditions, medication are the most primary reasons for bad breath.
HYGIENE
1) Brush your teeth twice a day. Brushing the teeth before bed is more important than brushing in the morning as the bacteria in the mouth if not removed before sleep will get the longest time to multiply and cause bad breath.
2) Brush at least for 2 minutes with brush at an angle 45 degrees. Clean every teeth individually and pay additional attention on the inner surfaces of molars.
3) Floss at least once a day.
4) Gargle after every meal and snack.
5) Change the brush every three months
6) Maintain a tongue scrapper. Scrap the tongue gently 5-6 times after you brush. Try to reach the back of your tongue with out gagging.
PERSONAL HABITS
1) Smoking leads to nicotine deposition on teeth and gums. It also leads to dry mouth.
2) stinky breath likes alcohol.
PRE EXISTING MEDICAL CONDITIONS 
Here are characteristic bad breath odors associated with some of the pre existing medical conditions

• Diabetes - acetone, fruity
• Liver failure - sweetish, musty
• Acute rheumatic fever - acid, sweet
• Lung abscess - foul, putrefactive
• Blood dyscrasias - resembling decomposed blood
• Liver cirrhosis - resembling decayed blood
• Uremia - ammonia or urine
• Hand-Schuller-Christian disease - fetid breath and unpleasant taste
• Scurvy - foul breath from stomach inflammation
• Wegner`s granulomatosis - Necrotic, putrefactive
• Kidney failure - ammonia or urine
• Diphtheria, dysentery, measles, pneumonia, scarlet fever, tuberculosis - extremely foul, fetid odor
• Syphilis - fetid

Source: http://www.silverbelldental.com/library/25/BadBreath(halitosis).html

MEDICATIONS

Few medications lead to dry mouth which causes bad breath. A comprehensive list of medications which cause dry mouth are enlisted in the following link

Medications that cause dry mouth