If there are symptoms, the following may indicate colon cancer: 1  Abdominal pain and tenderness in the lower abdomen. 2  Blood in the stool. 3  Diarrhea, constipation, or other change in bowel habits. 4  Narrow stools. 5  Weight loss with no known reason.
Diarrhea may be caused by: lactose intolerance. bacterial infection. damage to intestinal wall. hormone-producing tumors (including certain types of pancreatic neuroendocrine tumors) inflammation or ulceration of the bowel. malabsorption caused by surgery that has shortened the intestines. pancreatic enzyme insufficiency.
Bacteria, parasites, fungi, or viruses can cause the infection. 1  Parasites: The parasites cryptosporidium or microsporidium used to cause diarrhea in many people with HIV. The use of combination antiretroviral therapy has greatly reduced the rates of these problems. 2  Antiretroviral medications: These can cause diarrhea.