I thought I'd post an intersting animation I made a while back.   I occasionally make animations for educational purposes and you may have seen some on this or other websites I have had in the past.  If you are a patient who sees me at my Oakville practice, you have seen this and other dental education I play on the monitors in my office.
Here you can see a cavity start on a tooth that has no decay to start.  Just beside this tooth you can see two teeth with fillings...they look white.decay to abcess -Xray "movie"
As the decay progresses, it gets larger and deeper in a typical manner, eventually reaching the dark area in the center of the tooth.  This dark area in the center of the tooth is the "hollow" inside of the tooth where the "nerve" or dental pulp is.  The pulp is the living part of the tooth.  When the decay reaches the pulp, the bacteria that has caused the decay (destruction of the hard tooth structure) infects the pulp tissue and it dies.  The dead tissue breaks down and abcesses and on this Xray Animation you see a large round shadow form at root tip.  This dark spot is the breakdown or destruction of bone there by the abcessing nerve tissue. 
As you might guess,  I try to get the cavities when they are small and easy to repair.  Once the decay reaches the nerve, root canal therapy is needed and we want to avoid that.  This is one of the reasons we dentists take radiographs or "X-rays" at check ups... to get the cavities when they are small and easy to fix!