Obstetric fistulas are the only preventable fistulas. These can be prevented with good prenatal and childbirth care and by avoiding pregnancy in very young girls.
A fistula is an abnormal connection between an organ, vessel, or intestine and another structure. What happened to using google when you have questions like this?
Yes. Gangrene is preventable. Keep ALL body parts covered from cold weather. Wash cuts with peroxide or antiseptic bacterial soap before putting a bandaid on.