Bad breath affects 50 to 60% of the population. There are many causes of bad breath but the main ones are decayed teeth and diseased gums. Gingivitis or inflammation of the gums need to be treated right away. If not, the gum becomes red and angry, swollen and would lose its attachment to the tooth, thus developing a pocket around the tooth. Food gets trapped within the pocket, causing more irritation to the gum and bacteria accumulates within the pocket. The bacteria causes breakdown of the food creating compounds called volatile sulphur compounds (VSC).
It is this which is the source of bad breath.
Saliva is a strong antibacterial substance and in its absence, bacteria can grow. Dry mouth can be caused by sickness or is a side effect of certain medicines. Hence, it is important to treat it.
Eating onions and garlic, cheeses could contribute to bad breath.
Smoking, consumption of alcohol.
In sinusitis and tonsillitis where there is accumulation of mucus, bacteria can accumulate in the mucus and produce VSC, thus causing bad breath.
A weakened lower oesophageal sphincter between the stomach and the oesophagus can result in reflux of food back into the oesophagus, thus allowing gases to escape.