This introductory textbook was first published in 1978. It was originally intended primarily for students who wanted to obtain a simplified picture of what viruses are like and how they can multiply and cause disease. Today, it still provides a general overall picture of virology, emphasising the underlying fundamental biochemical principles rather than detailing the complexities of different viruses or clinical problems. The mechanisms of replication and the variety of structures found in viruses are dealt with from a comparative standpoint. A working knowledge of proteins, nucleic acids and lipids is assumed.