Blood mixed with stool is most often caused by upper gastrointestinal bleeding, but anal fistula is also not excluded. The most common cause of blood in the stool is upper gastrointestinal bleeding, which is characterized by blood mixed in the stool and the color of the blood is dark red or black. The main lesions include colitis and ulcerative enteritis, bleeding from the gastric mucosa, etc. The main cause of these conditions is bacterial or viral infection caused by poor diet. In addition, necrosis of cells or mucous membranes in the gastrointestinal tract can also cause this condition. Anal fissures around the anus can also cause bleeding from the fissure during defecation and cause blood to pass in the stool. The blood in the stool is usually bright red in color, and there is significant pain and itching. In addition to these causes, malignant tumors in the intestine can also cause blood in the stool.