To echo the other responses with a practical example, suppose you were to have a slow growing prostate cancer. Suppose that there was a 0% chance it'd actually

To echo the other responses with a practical example, suppose you were to have a slow growing prostate cancer. Suppose that there was a 0% chance it'd actually cause you any real harm. This is a not at all uncommon scenario. Now suppose an early screen detects this cancer. Since we still have difficulty discerning whether or not your cancer is really "bad" or not you'll likely undergo immediate treatment. This treatment leaves you feeling horribly ill for the duration with all sorts of side effects. After treatment you're left with urinary leakage and erectile dysfunction. I don't know about you but if you were to tell me that I could live the rest of my days with a minor cancer growing in my body minding its own business I'd choose leaving it there vs the side effects of the treatment.