Known to most people as tooth decay, caries is the most common disease in the world, affecting practically everyone to some extent. The cause is quite straightforward: there are natural bacteria which live in the human mouth, and which form a buildup on the teeth called plaque. Plaque is not in itself harmful, but when it interacts with sugary or starchy substances, it forms acids which eat away at the hard enamel surrounding the tooth, demineralizing it and causing the interior to decay. Once the enamel is gone, the process of decay accelerates in the softer interior of the tooth.
Quantity of sugary and starchy foods in the diet is therefore a major determiner for tooth decay, but there are other factors at work too, for example the daily intake of calcium and other minerals. Many countries insert fluoride into the water in order to remineralise teeth and counteract plaque acid, and these countries generally have lower rates of tooth decay given the average diet. Children and elderly people are most at risk for tooth decay, and there have been many campaigns in England and Ireland encouraging children to drink milk for this reason. Also, once there are dental fillings present in the mouth from previous cavities, the surrounding teeth are more sensitive to decay, because the plaque bacteria tend to congregate around them.
Saliva is a natural counteractive agent to plaque acid, and can also serve to remineralise teeth. This is why dentists sometimes recommend chewing sugar free gum to stimulate saliva production. However, practically all dentists also recommend brushing after every meal with fluoridated toothpaste and flossing every day to remove trapped pieces of food.
Caries is never a life-threatening disease, but can lead to considerable problems if not dealt with. Infected teeth can lead to infected gums (gingivitis) and infected root canals, which can be very painful and unpleasant (not to mention expensive to put right). Also, decayed teeth look bad and smell bad. There has been some controversy recently over the use of mercury amalgam in the filling of cavities left by caries, due to research which suggests that it may have be poisonous over a log period of time, and some people therefore opt for a slightly weaker white amalgam instead, but almost everyone would prefer a mouth full of metal to a mouth full of rotting teeth.
- Factoid no.1: caries is a disease transmittable by sharing a toothbrush, which may be why many people have such an aversion to this even when they may use their mouths for many activities which seem far 'dirtier'.
- Factoid no.2: dentists reputedly have one of the highest suicide rates of any profession (6.64 times the rate of the normal working population). Supposedly this is because no one likes going to the dentist, and because their area of study and work is so cramped. This may or may not be true - there's a good discussion of this possibly dodgy statistic at The Straight Dope: http://www.straightdope.com/columns/010420.html