The influence of basic science, particularly molecular biology, in human and veterinary medicine revolutionized thinking in many aspects and changed fundamentally and creatively the classical strategy for research and prevention of infectious diseases. Genetic engineering and related disciplines have progressed to a remarkable degree over the last decade and now form the keystone supporting medicine. These are strong and efficient instruments for health and disease oriented research and their application gives the opportunity to receive more answers and not only more questions. The prime objective of this book is to create new knowledge within the medical disciplines and inspire colleagues working in this field with the unity and unambiguous importance of this science and its technologies for identifying, clarifying and planning new strategies for curing and preventing disease. This book contains original studies on the molecular biology of animal viruses. Some of the viruses discussed in this book are also hazardous to man. In this light it can be considered as a contribution to modern education on the human infectious diseases. From this point of view the book contains a chapter on Hantaan virus that causes no detectable disease in animals but hemorrhagic fever with renal syndrome has been attributed to infection of humans by this virus.