This text provides an explanation of the basic and medical aspects of virology in a comprehensive format. It is comprised of selected chapters from the third edition of "Fields Virology" (ISBN 0-7817-0253-4) that have specific relevance for all medical and graduate students, as well as researchers in infectious disease and other clinical scientists. The book is divided into two major sections: the first addresses general principles such as virus taxonomy, analysis of viral membranes, defective genomes and pathogenesis of viral infection; the second section details specific virus families and their replication and provides critical updates on specific virus families.