TOOTH DECAY :
Tooth decay is a bacterial disease which result in dissolving of hard tissues (calcium & phosphate) of the tooth surface by weak acid production from fermentation of food (carbohydrate mainly sugar & a less extent the protein or fat).
By the end of this a cavity will form usually at smooth surfaces (contact areas between the teeth) and depressions on the biting surfaces (fissures and pits) and the cervical margins near the gum border (free gingival) because there surfaces are less exposed to saliva and by difficult be cleared by normal brushing, they need more attention during brushing.
The susceptibility of patients to tooth decay depends on may factors the most important one :-
1- The type of food taken.
2- The efficiency of tooth brushing
3- The difference between loss of part of calcified tissue of the tooth and the bone, the first one is irreversible, that means when it started it will not return to normal again and in this case decay should be removed completely and replaced by dental fillings otherwise it will progress and leave more loss of tooth structure and if left it will result in affecting the pulp tissues (which composed of nerves, bloods vessels and lymphatics) and end with infection of the pulp and the apical area.  This will require more advanced treatment and longer procedures (root canal treatment & crowns).