The problem is caused by sticky deposits called plaque that collect, in particular, around the gum line, the edges of fillings and the grooved surfaces of the teeth.
Plaque is made up of food debris, saliva and the bacteria that are normally present in the mouth, and convert food into acids.
If plaque is allowed to collect over time it will harden into a substance called tartar.
Both tartar and plaque contain acids which, over time, can dissolve away the protective, hard enamel coating of the tooth, and create holes, or cavities.
Most cavities form over a period of months, or even years.
They are usually painless, but they can grow very large, and damage the much softer internal structures of the tooth such as the dentin and the pulp, which is found at the core.
If they remain untreated, they can kill the nerve and blood vessels of the tooth, and ultimately the tooth itself.