What is epidemiology? What are the causes of a new disease? How can pandemics be prevented? Epidemiology is the study of the changing patterns of disease and its main aim is to improve the health of populations. It's a vital field, central to the health of society, to the identification of causes of disease, and to their management and prevention. Epidemiology has had an impact on many areas of medicine; from discovering the relationship between tobacco smoking and lung cancer, to the origin and spread of new epidemics. However, it is often poorly understood, largely due to misrepresentations in the media. In this Very Short Introduction Rodolfo Saracci dispels some of the myths surrounding the study of epidemiology. He provides a general explanation of the principles behind clinical trials, and explains the nature of basic statistics concerning disease. He also looks at the ethical and political issues related to obtaining and using information concerning patients, and trials involving placebos.