Perianal abscesses are caused by bacteria that are normally present on the surface of the skin around the anus. The bacteria become trapped in sweat glands or hair follicles, or enter the tissue beneath the skin when the skin becomes irritated. When the bacteria start to grow, the body forms an abscess to fight the infection.

Causes of irritation to the skin around the anus include:

• Anal injury
• Anal sex
• Crohn's disease
• Hemorrhoids
• Pinworms
• Pruritus ani