Colon cancer, or colorectal cancer, starts in the large intestine or the rectum. Colon carcinoma is the most common type of colon cancer, though other rare types exist. There is no single cause for colon cancer, but most slowly develops from noncancerous polyps. If you have Crohn's diease, you have a higher risk for developing colon cancer. Other risk factors include having cancer elsewhere, colorectal polyps, family history of colon cancer, personal history of breast cancer and ulcerative colitis.