Bacteria live in the mouth, generally at the back of the tongue. In most of the people, they are the main causes of bad breath. The mouth's warm, moist conditions are ideal for the growth of these bacteria. Most bad breath is caused by bacteria in the mouth. 

Bad breath can also be caused by the following:

Improper brushing and cleaning of teeth and mouth, which allows bits of food to decay inside the mouth

Infection of the teeth and gum called periodontal disease

Respiratory infections like Pharengytis, Sinusitis.

Some pungent food usage in food preparation like garlic, onions, coffee.

Some bad habits like cigarette smoking, chewing tobacco , alcohol drinks.

Dry mouth problem caused by salivary gland problems, medicines or mouth breathing

Constipation caused by not taking balanced food

Some chronic diseases like diabetes, liver disease, kidney disease, lung disease, sinus disease, reflux disease and other. 

What You Eat - How you smell?                            

Food you eat is broken down in your mouth. As food is digested and absorbed into your bloodstream in the intestines, they are finally carried to your lungs and given off in your breath. When you eat foods with strong odors such as, garlic or onions brushing and flossing - even mouthwash - merely covers up the odor temporarily. The odor will not go away completely until the foods have passed through your body.

Poor the Habits - Bad you Breath

If you don't brush and floss your teeth daily, food particles can remain in your mouth, which promotes bacteria growth between teeth, around the gums, and on the tongue. This causes bad breath. In addition, odor-causing bacteria and food particles can cause bad breath if dentures are not properly cleaned.

Smoking or chewing tobacco-based products can also cause bad breath, stain teeth, reduce your ability to taste foods, and irritate gum tissues.

Health Problems Are Associated With Bad Breath

Persistent bad breath or a bad taste in your mouth may be warning signs of gum disease. Gum disease is caused by the buildup of plaque on teeth. The bacteria cause toxins to form in the mouth, which irritate the gums. If periodontal disease continues untreated, it can damage the gums and jawbone.

Other dental causes of bad breath include poorly fitting dental appliances, yeast infections of the mouth and dental caries.

The medical condition dry mouth  (also called xerostomia) can also cause bad breath. Saliva is necessary to moisten and cleanse the mouth by neutralizing acids produced by plaque and washing away dead cells that accumulate on the tongue, gums, and cheeks. If not removed, these cells decompose and can cause bad breath. Dry mouth may be caused by the side effects of various medications, salivary gland problems, or continuous breathing through the mouth.

Many other diseases and illnesses may cause bad breath. Here are some to be aware of: respiratory tract infections such as pneumonia or bronchitis, chronic sinus infections, postnasal drip, diabetes, chronic acid reflux, liver or kidney problems.