Do you wonder how cavities actually form? Tooth decay begins with harmful bacteria in the mouth that feed on sugars and carbohydrates. If poor oral hygiene allows these bacteria to continue to grow without interference, the acid they produce (as a by-product of metabolizing sugar) can demineralize (dissolve) tooth enamel. Ordinarily, this process happens slowly, giving the body time to replenish (remineralize) the enamel. However, when populations of bacteria outpace the body’s ability to rebuild enamel, the first stage of tooth decay (dental caries) results, appearing as a spot discernable only to the dentist or on an x-ray. The dentist may stop this decay process by applying fluoride, but  once the decay has penetrated the tooth enamel, a cavity forms.

P.S. Fluoride and minerals in saliva help increase remineralization of teeth.