The Dental Student to Sara's Parents: "Today I examined Sara’s teeth. You were correct. Sara does have an area of decay. Sara’s area of decay or cavity, also referred to as a carious lesion, (plural form is caries), was probably due to plaque/biofilm remaining on the occlusal surfaces like it is in this magnified picture.
In addition, as you can see in the picture here, there are several areas in which decay can occur. The top image is that of occlusal decay. It can be tiny and is hard to determine how far it has advanced without an x-ray. It starts in the pits and grooves on the chewing surface. The middle image is of interproximal decay, or decay between the teeth. This decay is very difficult to detect without an x-ray. The bottom image is of root caries. It usually only occurs when the root of the tooth is exposed due to the gums recessing. We usually do not see this type of decay in a child of Sara's age.
Cavities begin on the enamel portion of the tooth usually as a "white spot," which you can see in the middle image of the drawing above. This is due to demineralization or breaking down of the enamel. If it is left untreated, it forms a cavity and the enamel cannot be repaired. At this point, it requires the decay to be removed and either an amalgam (photo to the left or composite (tooth colored) filling material be placed to restore the tooth to its full function. If decay is allowed to progress without intervention, it will cause more tooth structure to be lost and may require a crown to restore it. If it progresses into the pulp, it can cause an infection and require root canal therapy or possible extraction of the tooth."