The purpose of epidemiology is to study the frequency and cause of disease occur in different populations. We use epidemiological information to plan and evaluate strategies to prevent diseases. This information can also be applied as a guide to help us to manage those patients who are already ill. Unlike clinical studies that pay attention to the health of individuals, epidemiology studies care more about the health of populations. Epidemiology studies focus on the distribution and determinants of human disease and health outcomes, and the application of methods to improve human health. As a result, the nature of epidemiological studies is typically observational, and the researchers have limited control over the exposure that study participants experience.