The following is provided by the National Cancer Institute

Cancer of the colon or rectum is among the most common cancers in the United States. The National Cancer Institute estimates that more than 112,000 new cases will have been diagnosed in 2007. Colon cancer can occur in both men and women and is most often found among people over the age of 50, although it does occur in younger people, and even (rarely) in teens.

Colorectal cancers are linked to diets that are high in fat and calories and low in fiber; so good nutrition with a low-fat/high fiber diet is crucial to prevention, particularly if you have a family history of colorectal cancer.

Screening tests help your doctor find polyps or cancer before you have symptoms. Finding and removing polyps may prevent colorectal cancer. Also, treatment for colorectal cancer is more likely to be effective when the disease is found early. People in their 50s and older should be screened. People who are at higher-than-average risk of colorectal cancer should talk with their doctor about whether to have screening tests before age 50, what tests to have, the benefits and risks of each test, and how often to schedule appointments.

No one knows the exact causes of colorectal cancer. Doctors often cannot explain why one person develops this disease and another does not. Research has shown that people with certain risk factors are more likely than others to develop colorectal cancer. Studies have found the following risk factors for colorectal cancer:

Age over 50: Colorectal cancer is more likely to occur as people get older. More than 90 percent of people with this disease are diagnosed after age 50. The average age at diagnosis is 72.

Colorectal polyps: Polyps are growths on the inner wall of the colon or rectum. They are common in people over age 50. Most polyps are benign (not cancer), but some polyps can become cancer. Finding and removing polyps may reduce the risk of colorectal cancer.

Family history of colorectal cancer: Close relatives (parents, brothers, sisters or children) of a person with a history of colorectal cancer are somewhat more likely to develop this disease themselves, especially if the relative had the cancer at a young age. If many close relatives have a history of colorectal cancer, the risk is even greater.

Genetic alterations: Changes in certain genes increase the risk of colorectal cancer. Hereditary nonpolyposis colon cancer (HNPCC) is the most common type of inherited (genetic) colorectal cancer. It accounts for about 2 percent of all colorectal cancer cases. It is caused by changes in an HNPCC gene. Most people with an altered HNPCC gene develop colon cancer, and the average age at diagnosis of colon cancer is 44.

Familial adenomatous polyposis (FAP) is a rare, inherited condition in which hundreds of polyps form in the colon and rectum. It is caused by a change in a specific gene called APC. Unless FAP is treated, it usually leads to colorectal cancer by age 40. FAP accounts for less than 1 percent of all colorectal cancer cases.

Family members of people who have HNPCC or FAP can have genetic testing to check for specific genetic changes. For those who have changes in their genes, health care providers may suggest ways to try to reduce the risk of colorectal cancer, or to improve the detection of this disease. For adults with FAP, the doctor may recommend an operation to remove all or part of the colon and rectum.

Personal history of cancer: A person who has already had colorectal cancer may develop colorectal cancer a second time. Also, women with a history of cancer of the ovary, uterus (endometrium), or breast are at a somewhat higher risk of developing colorectal cancer.

Cigarette smoking: A person who smokes cigarettes may be at increased risk of developing polyps and colorectal cancer.