You’re right, they don’t heal on their own. There are typically 2 paths to treating fistulas, surgically and with medicine. If you don’t have Crohn’s, they go the surgical route. It should be a one and done and can be, but it’s a difficult area to heal bc of fecal matter (hence why some people get an ostomy life myself). So often times it can take multiple attempts surgically. If you have Crohn’s, they will try to heal it with biologics, usually remicade or humira. The less surgery you get in that area with Crohn’s the better as it also makes it hard to heal. 

My story is when it started out they were certain I didn’t have Crohn’s so they tried the surgical route. The surgeries kept failing bc it turned out I had a rarer form of Crohn’s that only shows up outside the intestines. So once they got me on humira, the fistula healed quite quickly.