A tooth basically has 3 layers; enamel, dentin, and pulp (the nerve of the tooth). The outer layer if the tooth (enamel) is the hardest substance in your body and mostly made of minerals. There are no blood vessels in the enamel (although your tooth is very much alive because of the pulp) and cannot regenerate itself once significant portions have been destroyed. When enamel starts to demineralize and “soften” (due to acid wear from bacteria (i.e. eating sugar, foods/ drinks with very high or low ph, throwing up) the tooth will try and defend itself by remineralizing, but if there is too much acid exposure too often (combined with poor oral hygiene and genetics) it can cause holes (called caries) to form. Since the enamel is not “alive” it is not able to renew/fix itself after a certain point. Usually during early stages, if decay is isolated only to the enamel and does not progress to the second layer of dentin (which is much softer and has a similar density to bone) it is possible to remineralize and “reverse” the cavity.
Your teeth can heal themselves. But as stated above, we get cavities when the healing process can’t make up for the destruction caused by bacteria and their byproducts.
Now, the healing in our teeth is a bit different from other tissues in our body, meaning that once a cavity has formed, the teeth won’t be able to regrow the lost parts, but may be able to stop the process going any further and form a ‘scar-like’ tissue.
There are three major factors that contribute to the development of cavities(Keyes triad):
Diet. In the modern world, people consume food that has been processed( from which mostly carbohydrates are responsible for allowing the bacteria to develop) and has the ability to stay on the teeth surfaces longer, providing a ‘good’ environment for bacteria.
Bacteria. Everyone has all sorts of bacteria in their mouths, but some people may have more of the pathogen bacteria involved in caries. Bacteria alone can’t do any damage, but since almost always it’s complemented by the other two factors, caries develop.
Host factor (i.e. tooth and saliva). There are many systems in our body to combat the negative actions done to our teeth, some present in our saliva, others in our teeth. So people that don’t secrete enough saliva, or the one secreted is ‘weak'(can’t manage to stop the demineralization process going on) will be more likely to develop caries. The same goes for our teeth structure. Some people don’t have strong enough enamel, due to something intervening in the development process.
So when these three factors are working togheter and the dental hygiene is also not so great, the body’s ability to stop the caries can’t make up for the damage done to the teeths. Once something changes in the Keyes triad or a better dental hygiene is established, the body has the ability to stop the dental caries ( if the process isn’t too advanced – meaning the bacteria reached the dental pulp, and from then on, the process is ireversible to the pulp, and without the dentists intervention an infection will continue to develop).