Root inflammations can have many causes, the most common one being cavities!
Some other possible causes are plastic fillings, excessive drilling when setting crowns, or after an accident – more details here.
If root treatment is not performed correctly, or complications arise, or if necessary treatment is not performed at all, then the dental nerve causes an inflammation in the bone, leading to a cyst in the root. This need not always be accompanied by pain, however. More on this topic here.
At some point the inflammation will make itself felt through the mucous membrane, leading to a fistula at the dental root. If the problem is not too big, root treatment may help to reverse the process, otherwise a root tip resection will have to be performed.
Inflammation of the root, fistula and resection – is this really necessary?,
I saw this video about tooth inflammation, it’s also very informative http://www.checkdent.com/en/videos/tooth-inflammation-159.html