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There are four types of influenza virus.

*Influenza A : Influenza A viruses cause seasonal flu epidemics practically every year in the United States. They can infect humans and animals.Influenza A is the only type that can cause a pandemic, which is a global spread of disease. Bird flu and swine flu pandemics both resulted from influenza A viruses. An influenza A virus has two surface proteins: hemagglutinin and neuraminidase. These help doctors with classification.