Family of RNA viruses containing four genera, some of which are noted for causing influenza in humans. Orthomyxoviridae have enveloped virions (virus particles) that measure between 80 and 120 nm (1 nm = 10−9 metre) in diameter. The nucleocapsid, which consists of a protein shell, or capsid, and contains the viral nucleic acids, has helical symmetry. The orthomyxovirus genome contains eight segments of single-stranded negative-sense RNA (ribonucleic acid), and an endogenous RNA polymerase is present for the transcription of the negative-sense strand into a positive-sense strand to enable protein synthesis. The lipoprotein envelope of the virion contains two glycoproteins, designated hemagglutinin (major antigen) and neuraminidase.
