The concept of viruses as a natural phenomenon separate fromother infectious agents is more than 100 years old. Recognitionof the fact that many diverse diseases of plants and animals canbe attributed to this newly recognized type of agent came morelately. So did the recognition of phages that kill bacteria. In morerecent years, the appreciation that practically all living speciesmay have viruses associated with them distinguish the widespreadprevalence of these agents and their potential importance in nature.Although not all viruses are pathogenic in their host, several haveraised great concern in the present century because of the seriousepidemic threats in humans like influenza, poliomyelitis, herpes,AIDS and childhood diarrhoea. Most texts do not attempt to coverthe entire field of virology because of the breadth of this subject.The present attempt was made to fill this gap as a prerequisite tothe use of this book.The chapters in this book describe the properties of the majorvirus families of current importance starting with bacterialvirology, then to plants, humans and finally attempts to providelatest information on antiviral drugs and viral vaccines. The virusfamilies of plants and humans are discussed in a systematicmanner. Properties of viruses, major viral diseases, mechanism oftransmission, prevention and control measures are dealt in detailin the text. As a means of fine-tuning the readers thought, a setof short questions are given at the end of each chapter for needfulanalysis of the subject. The purpose of this book is to introducethe reader to a wide informative data on viruses infecting bacteria,plants and animals, and to meet the requirements of a completematerial on Virology.