Here's a situation:

Tooth root canaled three years ago. In January some discomfort for a day, fistula developed. No discomfort; just a little sensitive to toothbrush. X-rays show nothing except small dot in center of tooth. Root canal retreated. Three weeks later permanent filling. Two months later either a new fistula, or the old one, is draining. What is, given the above facts, most likely causing the infection, and the fistula to keep draining?