The main cause of appendicitis is a blockage in the hollow part of the appendix. This blockage is most often due to a calcified "stone" made of feces called a fecalith or appendicolith. Other possible causes include inflamed lymphoid tissue from infections, intestinal parasites, gallstones, tumors, or other foreign bodies. When the appendix is blocked, mucus builds up inside, increasing pressure and reducing blood flow, which leads to inflammation, bacterial growth, tissue injury, and possibly tissue death. If untreated, this can cause the appendix to rupture, leading to severe infection or peritonitis.

Summary of causes:

• Blockage by fecalith (hardened stool or poop stones)
• Swelling from infection or lymphoid hyperplasia
• Parasites blocking the appendix
• Tumors or foreign bodies obstructing the appendix
• Inflammation spreading from nearby intestinal conditions

These blockages trap mucus and bacteria, causing swelling, infection, and potentially rupture.

Thus, appendicitis is primarily caused by obstruction of the appendix followed by infection and inflammation.