From HIV to influenza, the battle between infectious agents and the immune system is at the heart of disease. Knowledge of how and why parasites vary to escape recognition by the immune system is central to vaccine design, the control of epidemics and our fundamental understanding of parasite ecology and evolution. This text uses an evolutionary perspective to meld the terms and findings of molecular biology, immunology, pathogen biology and population dynamics. This multidisciplinary approach offers newcomers a readable introduction while giving specialists a guide to allied subjects. Every aspect of the immune response is presented in the functional context of parasite recognition and defence - an emphasis that gives structure to a large amount of data and brings into focus the complexity of immunology. The problems that end each chapter set the challenge for future research and the text includes extensive discussion of HIV, influenza, foot-and-mouth disease and many other pathogens.