Virology is the science concerned with the study of viruses and virus-like agents. Study of viruses encompasses an examination of their structure, evolution, classification, interactions and the diseases they cause. Virology also lays due emphasis on the techniques of isolating and culturing viruses, and investigating ways to use them for research and therapy. Virology is a subfield of microbiology and medicine. A major dimension of study in virology is virus classification. Viruses may be classified relative to the host cells that they infect, the shape of their capsid, the viral structure, the nucleic acid used as the genetic material or the viral replication method they employ. The structure and shape of viruses can be studied by NMR spectroscopy, electron microscopy and X-ray crystallography. Viruses cause several infectious diseases, such as measles, common cold, influenza, rabies, hepatitis, AIDS, polio, small pox, etc. Certain viruses may contribute to the development of some cancers. The use of viruses as gene vectors is actively being pursued for gene therapy. They are also being used to introduce new genes in host cells. Virology is an upcoming field of science that has undergone rapid development over the past few decades. This book presents this complex subject in the most comprehensible and easy to understand language. It is a vital tool for all researching and studying this field.