Hemorrhoids: This is often the most common cause of blood in stool, especially if the blood is bright red and seen on the toilet paper or in the bowl. Hemorrhoids are swollen veins in the lowest part of the rectum or anus, which can bleed due to increased pressure.

Diverticulosis: A condition where small, bulging pouches (diverticula) develop in the digestive tract, most commonly in the colon. These can bleed, especially if they become inflamed or if a blood vessel in a diverticulum bursts.
Anal Fissures: Small tears in the lining of the anus, which can cause pain and bleeding during and after bowel movements.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic inflammation and ulcers in the digestive tract, leading to bleeding.
Gastrointestinal Infections: Certain infections, such as those caused by bacteria, viruses, or parasites, can lead to inflammation and bleeding in the GI tract.

Colon Cancer: Although less common than other causes, colon cancer is a critical diagnosis not to miss. It can cause blood in the stool, which may be occult (hidden) or visible.
Upper GI Bleed: Bleeding from the upper gastrointestinal tract (e.g., stomach or duodenum) can lead to black, tarry stools (melena) rather than fresh blood. However, in some cases, especially if the bleeding is rapid, fresh blood can appear in the stool.
Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially cutting off blood supply and leading to ischemia and bleeding. This is more common in children but can occur in adults.
Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause bleeding, especially in children and young adults.

Angiodysplasia: Abnormal blood vessels in the colon that can cause bleeding, more common in the elderly.
Dieulafoy's Lesion: A rare cause of GI bleeding characterized by a small mucosal defect with an exposed, usually large, caliber artery at its base.
Blue Rubber Bleb Nevus Syndrome: A rare condition characterized by multiple venous malformations that can cause GI bleeding among other symptoms.
Osler-Weber-Rendu Syndrome (Hereditary Hemorrhagic Telangiectasia): A genetic disorder that leads to the formation of abnormal blood vessels, which can cause bleeding in various parts of the body, including the GI tract.