The immune system has evolved in large part to enable organisms to resist microbial infection. Microorganisms have long been used as experimental tools by immunologists, and the study of the immune response to viruses and bacteria has contributed much to our understanding of basic immunological mechanisms. There are also important practical and clinical reasons for attempting to understand the immunology of infections -- these include the rational design of vaccines, the pathogenesis of infectious diseases, the advent of AIDS, the rise in drug-resistant mycobacterial infections and the recognition of the infectious aetiology of peptic ulcer disease. The contributors to this book are all chosen for their active involvement and expertise in the fields. It bridges the divide between basic immunological research and clinical practice.