How does fistula usually form?

Inflammation: Either as a result of an infection that 'burrows' or from another source, e.g. Ibd (inflammatory bowel disease) - not to be confused with IBS (irritable bowel syndrome).

Inflammation, Cancer: Anything that can cause the intestine to rupture can potentially lead to a fistula. The most common causes include diverticulitis, inflammatory bowel disease, and colorectal cancer. When a hole occurs, the body's natural tendency is to want to seal this perforation. Typically, an adjacent organ will "band-aid" the hole shut, but the inflammation will burrow a hole into this second structure.