Optimal-intensity screening strategies focus on persons with sufficient risk for potentially fatal cancer who also have low competing health risks from other causes. Overdiagnosis is due to detection of not only slowly progressive cancer (Figure 3, patient 3) but also any type of cancer in patients with serious noncancer health risks that will end their life before the cancer becomes symptomatic (Figure 3, patient 4). An example is a hypothetical woman whose breast cancer is detected as a palpable lump (that is, by symptoms) at age 55 years and who would die of the cancer at age 65 years. With detection by screening at age 52 years and effective treatment, she would instead live until age 80 years and die of another cause, such as a stroke. Thus, her benefit (living 15 years beyond age 65 years) would come 13 years after she is screened at age 52 years. However, if this woman has severe congestive heart failure that would end her life at age 70 years, her benefit from breast cancer screening would be reduced from 15 years to 5 years. If her noncancer risk is even more serious (for example, if she has severe diabetes, end-stage renal disease, and cirrhosis), her life span may be decreased such that there is no benefit from breast cancer screening (Figure 3, patient 4). In fact, there may be unintentional harm if anxiety from and treatment of breast cancer reduce the quality and length of her life. This is another reason for the decrease in the slope of the benefit curve and the increase in the slope of the harms and costs curves with increasing screening intensity (Figure 1): The screening population is enlarged to include more persons with serious noncancer health risks.