Influenza A virus H3N2 is a subtype of viruses that cause influenza (flu). H3N2 viruses can infect birds and mammals. In birds, humans, and pigs, the virus has mutated into many strains. Hemagglutinin(HA) binds to sialic acid-containing receptors on the cell surface, bringing about the attachment of the virus particle to the cell. It plays a major role in the determination of host range restriction and virulence and is responsible for penetration of the virus into the cell cytoplasm by mediating the fusion of the membrane of the endocytosed virus particle with the endosomal membrane.