The main cause of anal abscess is perianal abscess or anal fistula. Perianal abscesses are divided into rectal submucosal abscesses, perianal abscesses, pararectal abscesses and abscesses in the sciorectal space and in the puborectal space. Among them, pararectal abscesses and perianal abscesses can be seen as red, swollen, and painful masses in the anus, and when the abscess cavity breaks down, there will be pus flowing from the anus. If pus comes out from inside the anus, it may be considered an abscess in the rectal space or a submucosal abscess in the rectum, and the pus cavity ruptures and pus flows. In addition, when an anal fistula is recurring or in the resting phase, the fistula can also be squeezed and the pus can flow from the anus.