The term dental decay refers to infected and broken down tooth
structure.  The process of teeth breaking down can be complex and
have many factors.  Put simply, the decay is caused by bacteria in the
mouth in the form of plaque.  The bacteria lives off of sugars and they in
turn produce acids that break down and weaken teeth.  If decay is
caught while it has only made a small cavitation or infected a small
percentage of the tooth, it can be restored with a
filling.  If the decay has
spread to the pulp of the tooth, a
root canal may be necessary.  Also,
once large portions of the tooth are missing, a
crown is usually
indicated.  As you can imagine, dental decay is one of the most
prevalent diseases in the world.
Dental Decay