The cause of an anal fistula is due to infection of the anal glands. In normal human anatomy, there is a location at the junction of the rectum and the anal canal called the anal sinus or anal gland, which is relatively weak. If it is damaged, intestinal bacteria can enter the anal gland, causing inflammation. Over time, the infected anal gland leads to further abscesses towards the perianal space, causing a perianal abscess. After the perianal abscess ruptures, it further develops into an anal fistula.