A fistula around the bottom is a tunnel that runs from the skin to inside the bowel, which may harbour bacteria.  It may result from an abscess around the anus that has persisted, but in turn may cause persisting infection.  It may act like a dormant volcano, so treating it will stop further erruptions. 

I invented the first hollow thread used in this fistula tunnel that shows on MR scans.  Ultimately the thread may be removed and a plug may be used to block the inside opening.  Please click here for more information regarding fistula plug treatment.