Hemorrhoids: This is the most common cause of perianal swelling, especially if the swelling is associated with pain, itching, or bleeding during bowel movements. Hemorrhoids are swollen veins in the lowest part of the rectum or anus.

Perianal Abscess: An infected cavity filled with pus that can cause significant pain and swelling around the anus. It often requires drainage.
Anal Fissure: A small tear in the lining of the anus, which can cause pain and bleeding during bowel movements, and may lead to swelling.
Perianal Fistula: An abnormal connection between the inside of the anus and the skin around it, which can cause recurring infections and swelling.
Proctitis: Inflammation of the lining of the rectum, which can cause swelling, pain, and discharge.

Rectal Cancer: Although less common, rectal cancer can present with perianal swelling, especially if there is a significant mass effect. Early detection is crucial for treatment outcomes.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause perianal disease, including swelling, fistulas, and abscesses, and require specific management.
Necrotizing Fasciitis: A rare but life-threatening infection of the skin and soft tissues that can start around the anus and spread rapidly.

Pilonidal Sinus or Cyst: A type of skin infection that typically occurs near the tailbone, at the top of the cleft of the buttocks, but can occasionally be found in the perianal area.
Hidradenitis Suppurativa: A chronic skin condition that can cause abscesses and scarring, sometimes affecting the perianal area.
Tuberculosis: Rarely, tuberculosis can affect the perianal area, causing swelling, fistulas, or abscesses.
Actinomycosis: A rare bacterial infection that can cause abscesses or masses in the perianal area, among other locations.