The complexity of disease causation needs to be clarified, taking into account the importance of approaching it from different perspectives. One concern put forth is that risk factor epidemiology might be forcing epidemiology to focus more on individuals and less on populations and public health, however public health is influenced by population level and individual-level determinants. Hence, the global context of population health comprises a variety of systems operating at various levels, and each constitutes the environment for the other. To achieve One Health is necessary to design studies that take into account the trajectories of health and disease over the life course, the effects of social contexts, the spread of communicable diseases and behaviors through populations, genetic causes, and historical trends. Eco-epidemiology recognized how social and biological changes could both interact to lead to disease that will be necessary to face public health challenges of our time.