This textbook provides an introduction to modern virology, focusing on the new molecular approach. It explains the principles and fundamental aspects of virology, including structure of virus particles and virus genomes; virus replication; control of gene expression; and virus pathogenesis (including AIDS). Later chapters consider the major current concerns in virology, such as responses to, and treatment of, virus infections, and the emergence of novel infectious agents, including viruses, viroids and prions. The summaries at the end of each chapter are designed to provide a useful revision aid.