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In order for screening to work disease needs to develop in a certain
pathway, or natural history. Healthy individuals go through a state
during which the disease, or a precursor to the disease is detectable,
prior to the actual onset of symptoms. When screening is possible
during this phase, it is referred to as the detectable preclinical phase.
In order for screening to improve health, intervention is required that
either averts the development of the disease or treats the disease in
such a way that advanced disease (such as metastasis of cancer) or death
due to the disease does not occur.
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