<unk>  influenza virus is usually transmitted from asymptomatic carrier pigs to humans. Novel H1N1 viruses are thought to have evolved from older influenza viruses by reassortment of formerly triple-reassortant<unk>  flu viruses. Influenza virus contains hemagglutinin, neuraminidase, non-structural proteins, matrix proteins, polymerase proteins, and nucleoprotein that are responsible for viral pathogenesis in humans . The HA protein on the viral surface functions as a receptor binding site and binds to host receptors that contain sialic acid to allow viral fusion to the host cell in the respiratory tract. Following fusion, viral replication typically takes place within 1 day, and polymerase proteins and nucleoproteins are involved in viral replication, whereas the matrix protein is responsible for viral assembly prior to viral release via cytolytic or apoptotic mechanisms. Viral proteins are, at least in part, responsible for down-regulation of cytotoxic T-cell activity, evasion of immune responses, and activation of cytokines and pro-inflammatory mechanisms that contribute to host tissue injury.<unk>  influenza undergoes antigenic drifts and shifts that ultimately result in genetic reassortment and capacity to reinfect the same host.
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