Fluoride exerts its major effect by interfering with the demineralization mechanism of tooth decay. Tooth decay is an infectious disease, the key feature of which is an increase within dental plaque of bacteria such as "Streptococcus mutans" and "Lactobacillus". These produce organic acids when carbohydrates, especially sugar, are eaten. When enough acid is produced to lower the pH below 5.5, the acid dissolves carbonated hydroxyapatite, the main component of tooth enamel, in a process known as "demineralization". After the sugar is gone, some of the mineral loss can be recovered—or "remineralized"—from ions dissolved in the saliva. Cavities result when the rate of demineralization exceeds the rate of remineralization, typically in a process that requires many months or years.