This book is about how plants get diseases, from the origins and evolution of parasites to how the great plant epidemics developed. Conditions favoring disease are inherent in agriculture and diseases became destructive because of human activities. This book also deals with how people have dealt with plant diseases in history, and includes the natural histories of some of the most damaging plant diseases worldwide, with discussions of why each became destructive. It classifies diseases according to the most significant factors in the development of epidemics: every case involves a human factor. Each model disease proceeds from observable facts to more complex concepts.