This book is an introduction to epidemilogy and to the epidemiologic approach to problems of health and disease. The basic principles and methods of epidemiology are presented together with many of the applications of epidemiology to public health and clinical practice. The book is organized in three sections: Section 1 Focuses on the epidemiologic approach to understanding disease and to developing the basis for interventions designed to modify and improve its natural history. Chapter 2 discusses how disease is transmitted and acquired. The measures we use to assess the frequency of disease are presented in Chapter 3. Chapter 4 addresses the critical issue of how to distinguish persons who have a disease from those who do not and how to assess the quality of the diagnostic and screening test used.