An anal abscess is a condition that occurs when there is a buildup of pus near the anus. They can be quite painful. The most common cause of anal abscesses is an infection that develops in the anal glands. The most common type of anal abscess is a perianal abscess, which looks like a swollen boil near the anus. These types of abscesses usually require treatment involving drainage and surgery. Treating the abscess this way often results in successful results. However, complications may arise. After draining an abscess, about fifty percent of patients develop a fistula.