Just inside your anus there are several glands that make fluid. Sometimes, they get infected blocked.or clogged. The infected tissue with liquid punches a hole in the skin somewhere near your anus to drain. The fistula is the tunnel that connects the gland to that and usually is in continuity with one or more external openings. Ayurveda calls this as BAGANDHARAM. It is a challenging disease because of its recurrence especially, and with deep roots.