Mechanisms of pathogenesis and disease progression. (a) The reductionist concept is that disease occurs as a consequence of a pathogenic factor, for example a microbe (top). This is overly simplistic. Disease is a consequence of susceptibility to pathogenic factors as well as exposure to them (bottom). The emergence concept provides important opportunities for better understanding disease risks, tracking pathogenesis and earlier intervention . (b) Disease progression is shown from baseline risk to irreversibility. Diseases develop as a consequence of inherited susceptibilities and environmental exposure. Over time, pathology increases, reversibility decreases and costs increase (red line). Earlier intervention could clearly reduce the costs and the disease burden .