Screening is looking for a specific condition in people who do not report symptoms of that condition. Doctors screen patients for cancer so that cancer may be found while it is still small and has not spread. Treatment at this early stage offers a better chance of a cure. However, for several years after starting screening, the cancer death rate in screened patients is often similar to that of unscreened patients. This is because the increased survival from screening is due to detection and treatment of early-stage cancer—cancer that would not have resulted in death for at least 5 years if it had not been detected by screening. As a result, patients with a life expectancy of less than 5 years when screening detects a small cancer are unlikely to extend their lives by screening. For instance, testing for blood in the stool to screen for cancer of the colon or rectum (colorectal cancer) reduces the cancer death rate only after about 4 years of screening. If patients die of other diseases, such as heart disease, during the time when screening doesn't alter the cancer death rate, screening hasn't done them much good. When a person's age and disease burden make it likely that they will die before screening reduces the cancer death rate, screening probably isn't worthwhile.