Influenza

Influenza is caused by infection of the respiratory tract with influenza viruses, RNA viruses of the Orthomyxovirus genus. Influenza viruses are classified into 4 types: A, B, C, and D. Only virus types A and B commonly cause illness in humans. Influenza A viruses are further classified into subtypes based on 2 surface proteins, hemagglutinin (HA) and neuraminidase (NA). Although 4 types and subtypes of influenza virus cocirculate in humans worldwide (influenza A(H1N1), A(H3N2), and influenza B-Yamagata, B-Victoria viruses), the distribution of these viruses varies from year to year and between geographic areas and time of year. Information about circulating viruses in various regions can be found on the CDC website (www.cdc.gov/flu/weekly) or the World Health Organization website (www.who.int/influenza/surveillance_monitoring/updates/latest_update_GIP_surveillance/en/). Avian and swine influenza viruses can occasionally infect and cause disease in humans, usually associated with close exposure to infected animal populations. Notably, avian influenza A(H5N1) and A(H7N9) viruses, as well as swine-origin A(H1N1), A(H1N2), and A(H3N2) variant viruses, have led to sporadic human infections globally.