Teeth, bacteria and food/drink are the only things you need to get cavities. There are certain factors that will increase your susceptibility to cavities. Factors like dry mouth, medications, poor oral hygiene, frequent snacking or sipping, diet and certain medical conditions are just a few that can lead to the breakdown of tooth structure.
These factors provide an environment that allow bacteria to thrive. Bacteria attaches to the tooth and produces acid. This acid causes tooth structure to breakdown and ultimately causes cavities.
Root surfaces are more prone to decay due to the absence of enamel. They are more porous (for lack of a better word) and are normally covered by bone and gums. Root exposure increases the likelihood of cavities and meticulous oral hygiene is a must.
Cavity formation can be a fast process depending on certain habits and factors. Radiographs (x-rays) help detect cavities as early as possible. Without routine x-rays, cavities can become extensive and most people will not have any symptoms until it is too late. Lack of pain is not a good indication of healthy teeth. In many cases, the extent of decay can cause the tooth to need substantial work to “fix” the tooth or even render it unsalvageable by the time symptoms develop.