Colitis is an inflammatory reaction of the colonic mucosa. Because the colonic mucosa exudes more fluid due to inflammation and the normal ability of the colonic mucosa to absorb water is diminished, stools are often very watery, which means that diarrhea occurs. Inflammation leaks more mucus, so mucus stools occur. The inflammation causes erosions and ulcers that lose blood, so there will be bloody stools, and if the inflammation involves larger blood vessels, there will be blood in the stool. Diarrhea is the most common clinical symptom of colitis. From thin watery stools, mucus stools, purulent stools, to bloody stools, the distinguishing properties vary depending on the degree of mucosal lesions. The frequency also varies. The frequency varies from one to dozens of times per day. The common symptoms of colitis also include abdominal pain of varying degrees, usually in the lower abdomen, especially in the left side of the lower abdomen, which can disappear on its own with defecation, and can be aggravated by cold or by eating a diet that is greasy and not easily digestible and that I do not tolerate. Other symptoms include abdominal distension; patients with significant inflammation of the rectum may experience shortness of breath, urgency and frequency of stools. In the acute phase of inflammation, there will be fever, and in long-term chronic diarrhea, there will be wasting and malnutrition. Those with significant blood in the stool will develop anemia. Those with severe diarrhea, blood in the stool, or symptoms such as anemia and fever need further tests including colonoscopy to clarify the diagnosis.