Influenza viruses are the viruses that cause influenza. These viruses can be categorized into four types: A, B, C and D. Influenza A and B viruses cause seasonal epidemics of flu and influenza A viruses are the only viruses that cause pandemics of flu disease. Influenza A pandemic occurs when a very different strain of influenza A virus emerges and has the ability to infect people and spreads rapidly between people. Influenza C virus infections do not cause human flu epidemics and cause mild illness. Influenza D viruses usually affect cattle. There are two proteins present on the surface of influenza A virus, namely, hemagglutinin (H) and neuraminidase (N). There are 18 different subtypes of hemagglutinin and 11 different subtypes of neuraminidase. This book provides comprehensive insights into the field of influenza virus. It is a valuable compilation of topics, ranging from therapeutics to vaccines of influenza virus. Those with an interest in this field would find this book helpful.