A fistula is an abnormal connection between two areas. These are thought to form when inflammation extends into the area surrounding the bowel wall. Fistulae can develop between two gastrointestinal organs (for example, the small and large intestines) and other organs like the bladder, vagina, and skin. The most common location for the fistulae is the ileum or the last part of the small intestine. A unique kind of fistula in Crohn's disease is a perianal fistula, which connects the inside lining of the rectum (the last part of the colon) with the skin surrounding the anus.

Depending on which organs are connected by fistulae, patients can have different symptoms. 

• Fistulae between the small and large intestine can cause diarrhea or passage of undigested foods. 
• Fistulae between the intestine and bladder can result in urinary tract infections, with symptoms of burning with urination, cloudy urine, or blood in the urine. 
• Fistulae to the vagina can result in passage of gas or stool through the vagina. 
• Finally, fistulae to the skin can initially present as a painful bump or boil (called an abscess) that then opens up and drains fluid or stool. 
• To treat fistulae, there are medical options (if there is also a significant amount of inflammation, oftentimes including antibiotics) and surgical options (especially if strictures recur frequently).

References:

1. https://www.crohnscolitisfoundation.org/emr/emr-ibd-education-tool/fistula