Sudden diarrhea with blood in the stool is considered to be caused by the following conditions: firstly, it may be due to excessive diarrhea, resulting in abnormal damage to the acute intestinal mucosa, which is most likely to lead to diarrhea with blood in the stool if the intestinal mucosa is already congested and edematous. Secondly, the symptoms of hemorrhoids may be aggravated by diarrhea, resulting in enlarged, edematous, painful, itchy, bleeding hemorrhoids, which are brightly colored and do not blend in with the stool. Then it may be a case of bleeding polyps due to repeated diarrhea, which increases the size of the polyps and prolapses, but the bleeding is usually not large. In addition, the bleeding may also be caused by excessive diarrhea that tears the skin of the anus, or it may be caused by upper gastrointestinal bleeding.