Influenza Virus: Therapeutics and VaccinesAbout This Hardback 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