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There are four types of influenza viruses: A, B, C, and D. In the United States, influenza A and B viruses cause seasonal epidemics of illness (known as flu season) in humans almost every winter. Influenza A viruses are the only influenza viruses known to cause influenza pandemics (i.e. worldwide epidemics of influenza disease). A pandemic can occur when a new and different influenza A virus emerges to infect humans with the ability to spread efficiently in a population against which people have little or no immunity. Influenza C virus infection usually causes mild illness and is not thought to cause human epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in humans.