In order for tooth decay to be developed in a tooth, that tooth must have acid producing bacteria around it, along with food for the bacteria to feed upon. Teeth that are susceptible to decay will have little to no fluoride in the enamel to fight the plaque. Fluoride can destroy decay, although it wont be able to do much once the decay has started to eat the teeth.
Poor hygiene habits will allow the plaque and tartar to build up around teeth and speed up the process of decay. Even though your mouth has a lot of bacteria that is always present, only one type will generate the acid that results in tooth decay. Some people have active decay that is always present in their mouths. Parents with active decay can easily pass the decay on to a child or loved one through eating, drinking from the same glass, or even kissing.
Once the decay has settled in the tooths enamel, it will progress very slow. Once it has made it through to the second layer of the enamel, it will spread faster as it heads towards the pulp. The pulp is a vital area of the tooth, as it contains the nerves and blood supply. This is where the pain will be the most intense, as the decay will start to eat...