Dental caries, more colloquially known as cavities, are holes in the harder tissue of teeth that expose the softer underlying tissue. They are caused when the bacteria that naturally occur in the mouth secrete acidic compounds as a natural product of their metabolism. The acid weakens and demineralizes the surfaces. Dental caries are probably the most common disease worldwide.
Although asymptomatic at first, if left untreated they will cause tooth discoloration, pain, and abcess.
Modern treatment is to drill out the diseased part of the tooth and re-fill it with acid resistant material, usually an alloy containing mercury for teeth that bear a heavy load (like molars) or a ceramic compound (where cosmetic considerations are more important, such as on front teeth). However, prevention is also important. Dentists recommend avoiding sugary or acidic foods, regular brushing with a fluoride based toothpaste, and flossing to remove the plaques that form between the teeth that harbor bacteria.