Epidemiology is the branch of medicine that studies how and why dierent groups of people have dierent risks of developing disease and the factors that are responsible for these dierences. It is the cornerstone of public health and, by identifying risk factors and targets for disease prevention, it provides the basis for policy decisions and evidencebased disease control and prevention. Epidemiological data on changes in disease rates over time can also be used to monitor the impact of primary prevention strategies.