The purpose of screening tests is to detect disease when it has not yet caused harm. For example, cancer of the large bowel (colon cancer) is detectable well before it acquires the ability to invade the rest of the body. Removal of the cancer at an early stage greatly reduces the likelihood that the patient will die of colon cancer. However, this benefit of screening does not occur immediately. It often takes many years between when cancer could be first detected by screening and when it would have grown large enough to cause symptoms or cause the patient's death. During this period, the patient feels the same as if the cancer had not been removed. If a person who is treated for a small tumor detected by screening dies before the cancer would have caused symptoms if it had not been removed, they have not benefited from screening. As people get older and develop serious diseases, they become less likely to live long enough to benefit from early detection and treatment of cancer, and they are more likely to experience complications from tests and treatments. Therefore, most doctors agree that cutting down or stopping screening tests is a good idea when a person has only a few years to live.