Sick individuals and sick populations

Aetiology confronts two distinct issues: the determinants of individual cas es, and the determinants of incidence rate. If exposure to a necessary agen t is homogeneous within a population, then case/control and cohort methods will fail to detect it: they will only identify markers of susceptibility. The corresponding strategies in control are the 'high-risk' approach, which seeks to protect susceptible individuals, and the population approach, whi ch seeks to control the causes of incidence. The two approaches are not usu ally in competition, but the prior concern should always be to discover and control the causes of incidence.