Decay results from the breakdown of enamel due to acids produced by dental plaque or bacteria that acts on the sugars present.
It can easily occur on root surfaces as they are much softer. One prevention is to brush regularly to prevent build up of plaque and to brush correctly so the gums do not recede exposing the cementum which lines the roots.
Decay may occur in grooves and pits of enamel surfaces, usually on the molars and premolars. Easily prevented by use of sealants to seal these vulnerable areas. Sealants are flowable resins that can be bonded into these biting surfaces.
Decay tends to occur under very old fillings especially in old silver amalgam ones. If the margins shows 'creeping' or raised, it is advisable to remove and check for decay and replace with tooth color composite fillings which is the current accepted standard.