How invasive is surgery or for a gastrointestinal fistula?

Invasive: It is major abdominal surgery. Generally it is done laparoscopically or as an open procedure, and can be very invasive.

It varies: A fistula is a communication between the bowel and another surface, can be skin or another organ. It really depends on the underlying disease process. They are often seen in crohn's diesease, cancer, post-surgical complications and many states in which the patient is immunocompromised and sick and/or on steroid which can make any surgery more complicated.