The causative agent of influenza; (in later use) specifically any orthomyxovirus of the genera Influenzavirus A, Influenzavirus B, and Influenzavirus C.
Human influenza is caused by type A and type B influenza viruses; type A viruses also infect other mammals and birds. Type A viruses are responsible for major epidemics and pandemics of influenza, which occur after a sudden major change in the viral surface antigens haemagglutinin and neuraminidase (antigenic shift), often as a result of reassortment between avian and human or swine viruses. In both type A and type B viruses these antigens also continually undergo minor changes (antigenic drift), forming many different viral strains, and allowing them to evade the immune response of previously infected hosts.
Late 19th century. From influenza + virus.