Overdiagnosis: how cancer screening can turn indolent pathology into illness.

The shift from illness to disease has had a profound impact on modern medicine - particularly in the realm of cancer screening. In screening, it is not patients with illness who seek help from the healthcare system; it is asymptomatic healthy individuals who are invited into the healthcare system to be examined for pathology. The underlying assumption of screening is that abnormalities and pathology always progress. If this were true, it would always make sense to look for disease even when people feel well. The million (or more accurately multi-billion) dollar question is whether the fundamental assumption that disease invariably leads to illness is valid. This is the question that the present paper will try to explore and answer.