Epidemiology is the study of factors that influence the health and illness of populations. The three levels of causation of health problems include individual behavioral level, individual biological level, and political-economic ecological level. Epidemiologists aim to understand the causes of health problems by looking at the relationship between agents, hosts, and environmental factors that affects health. Using these information, they also design public health interventions to solve various health problems among populations. They regularly evaluate the health of populations, tries to identify which populations have greater risk from specific causes of diseases, and evaluates the effectiveness of the intervention programs or methods that they create. In essence, epidemiology provides the backbone or foundation of public health and preventative medicine practices. 