The fishy smell of diarrhea is related to dysentery, ulcerative colitis, and intestinal tumors. Dysentery is mainly caused by the infection of the intestinal tract by the bacillus dysenteriae. When the bacillus dysenteriae invades the intestinal mucosa, it may even cause necrosis of the intestinal mucosa, and it is discharged together with the stool, which may cause diarrhea and a fishy smell in the stool. Ulcerative colitis is related to immune factors. After the disease, there will be ulcers on the mucous membrane of the colon, and some people will also have blood leakage. When intestinal tumor enters advanced stage, because of the invasion of cancer cells to the surrounding area, it will lead to local protein erosion and ulceration, which will not only lead to change of bowel habit and diarrhea, but also the erosion material will be discharged with the stool, so the discharge will have a fishy smell of corruption.