What makes us ill? How do we know what makes us ill? Biologists, doctors, statisticians, sociologists and historians all study health and disease. This book describes the basic methods of investigation used by all these professional, and shows how they are related and how they differ. The book works through examples such as diabetes and spina bifida to demonstrate how the causation of disease can be studied at many different levels - from microscopic processes within body cells to large-scale matters like the relationships between different groups of people in society. The aim is to provide readers with a critical understanding of the ways in which our knowledge of health and disease was arrived at, and to show how knowledge from various disciplines can fit together to provide a full picture of health and disease in our world.