There are many causes of bad breath but the main ones would be decayed teeth and diseased gums.
Red and swollen gums need to be treated right away or they would lose their attachment to the teeth, thus developing pockets around the teeth. Food gets trapped within the pocket and bacteria accumulates in it, causing breakdown of the food. This creates compounds called volatile sulphur compounds (VSC) which it the source of bad breath.
The removal of sub gingival tartar through curettage and root planning of the teeth followed by irrigation with hydrogen peroxide, which kills the anaerobic bacteria, will most certainly improve the condition of the gums.
This can only be done by the dentist as the offending agent is beneath the gum line.
The tongue, especially the upper back part, is relatively dry and usually poorly cleansed. It is coated with a film of food debris and mucus and bacteria in the mouth thrive well in such an ideal habitat.
Food gets trapped in between teeth and if not dislodged will start to rot in the mouth, thus causing bad breath. Hence, maintaining proper oral hygiene such as brushing the teeth, flossing, gentle cleaning of the tongue twice daily to remove the biofilm of bacteria will certainly reduce the bad odour.
Avoid certain foods such as garlic, onions, cheeses and alcohol. Smoking too can contribute to bad breath.
Dentures need to be cleaned properly and then soaked overnight in an antibacterial solution.
Chewing sugarless gum can help too as it promotes saliva production which then washes away the bacteria.
Mouth rinses which reduce the amount of plaque and bacteria in the mouth can also help reduce bad breath. However, they are only a temporary fix as they do not remove the source.