Epidemiology is the study of how often diseases occur in different groups of people and why. Epidemiological information is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom disease has already developed. Like clinical findings and pathology, the epidemiology of a disease is an integral part of its basic description.A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk. The population at risk is the group of people, healthy or sick, who would be counted as cases if they developed the disease being studied. Implicit in any epidemiological investigation is the notion of a target population about which conclusions are to be drawn. Occasionally measurements can be made on the full target population. More often observations can only be made on a study sample, which is selected in some way from the target population. How well the sample observations describe the target population depends on how representative the sample is of the target population.Another task of epidemiology is monitoring or surveillance of time trends to show which diseases are increasing or decreasing in incidence and which are changing in their distribution. This information is needed to identify emerging problems and also to assess the effectiveness of measures to control old problems.