Firstly, some definitions. Approaches to disease prevention are often classified at three levels:
Primary prevention is the approach that concentrates on minimizing risk factors in the community with the intention of minimizing the number of cases of the disease that arise in that community, namely reducing the incidence of disease. If effective at an affordable cost, this is clearly the best approach in terms of both public and personal health gain. It is reasonable to regard efforts to improve population health and resistance to disease (e.g., by healthy eating and physical exercise) as another form of primary prevention.
Secondary prevention refers to the detection of cases of the disease in question at an early stage in its natural history at which intervention is likely to lead to cure or to minimize morbidity and reduce eventual mortality. This is the category that encompasses screening. Screening is a complex area of science and the risks and benefits need careful evaluation in every situation. It is also reasonable to regard the prescription of dietary supplements, particularly antioxidants and some anti-inflammatory agents, as a form of secondary prevention.
Tertiary prevention refers to interventions designed to reduce recurrence of disease after treatment or to minimize the morbidity arising from treatment. Some of the approaches discussed under secondary prevention have application to patients already affected by the disease(s) in question.