Myth: Cavities are not contagious
Incorrect! Cavities are contagious because they are caused by bacterial of a certain class. When those bacteria are passed from the mouth of one person to another infection spreads. Newborn babies do not have any such bacteria, but in most cases a baby gets these cariogenic bacteria from its parents or caregivers. The most common modes of infection transmission will be sample testing milk or food for temperature, and kissing. Mothers may put their fingers inside the mouth of the baby to see if it has taken in some harmful object, or to feel for erupting teeth. If the fingers are pre-infected by the saliva of the mother the baby will also be infected. It is a fact that almost all adults do have cariogenic bacteria in their mouths acquired through any of these routes.
Myth: Sugar is the reason for cavities
It is a myth to the extent that sugar is not the only factor and that sugar does not cause cavities by itself. Cavities are caused by several cariogenic bacteria. But sugar (along with other carbohydrates) does provide an excellent environment which facilitates decay of teeth. Actually every one carries a biofilm of bacteria on their teeth. Some of the species present are cariogenic while others are not. Excess carbohydrates lower the pH of saliva which favors the multiplication of cariogenic bacteria, leading to demineralization of the enamel and tooth cavities.
Myth: Fluoride stops cavities
Fluoride will not stop formation for cavities. Fluoride actually provides an environment which promotes remineralization of enamel. To that extent it will reduce the chances of caries in children. But there is an unsettled debate whether fluoride can reduce the incidence of caries in adults.
Myth: Weak enamel causes cavities
This is neither completely true, but not entirely baseless. Actually, it is careless wording. Caries formation is the physically visible effect of a long slow process promoted by an acidic environment in the mouth. When the pH in the mouth drops below about 5.5 (getting more acidic) natural remineralization is not sufficient to cancel the demineralization of the enamel. The enamel is slowly eaten away, and will ultimately result in caries. So caries is actually a physical manifestation of the wearing away of the dental enamel.
Myth: Filling cures the disease
This is a very deceptive myth about dental caries. As mentioned above, cavities are a physical consequence of the disease. The disease is the predominance of the harmful bacteria. By restoring a cavity, the dentist is only alleviating the result of the disease. If the root cause is not treated, cavities will keep forming, and the dentist will keep filling them. The disease will stay. It is true that the cariogenic bacteria cannot be altogether eliminated, but their activity can be mitigated by proper hygiene, and treatment.
Myth: Brushing and flossing will cure caries
Brushing and flossing will not cure caries per se. But the importance of oral hygiene, especially of regular brushing and flossing cannot be over emphasized. We must again look into the mechanism of caries development. Two physical factors are essential for incidence of caries: cariogenic bacteria and acidic environment in the mouth. Add the invisible factor of time to this duo. So, availability of an acidic environment in the presence of bacteria for an extended period of time will have a cumulative effect towards dental decay. Regular brushing and flossing will reduce the time extent, and hence strongly inhibit cariogenic processes.