Influenza viruses are segmented, negative-sense RNA viruses belonging to the Orthomyxoviridae family. There are four types of influenza viruses: A, B, C, and D. However, types A and B are primarily responsible for seasonal flu epidemics in humans. Influenza A viruses are further classified into subtypes based on proteins on their surface, hemagglutinin (H) and neuraminidase (N). The most common subtypes in humans are H1N1 and H3N2.
