A bacterial accumulation causes a swelling pocket of infected tissue and liquid when the fluids secreted in the anus become obstructed or clogged. Most people refer to it as an abscess. If the abscess is not treated promptly, it will eventually travel outside and puncture a skin hole close to the anus to allow the pus to escape. The tunnel that runs from the gland to that orifice is called a fistula. Anal fistula is primarily caused by an abscess. On the other hand, the anal fistula can also result from illnesses including TB and STDs.
For anal fistula, there is no medicine available. Surgery is the only way to treat the fistula.