An anal fistula is a pathological channel that forms a connection between the anal canal, rectum, and the skin around the anus. It primarily develops from an infection causing a perirectal abscess around the rectal anal canal. These infections are generally purulent, with a smaller number due to tuberculosis. Other specific infections, such as Crohn's disease or ulcerative colitis, can also lead to anal fistulas. Generally, such infections are caused by Escherichia coli, leading to purulent infections; tuberculosis can be contagious, but generally, it is not infectious or contagious.