The Orthomyxoviridae (Derivation of name: orthos is Greek for straight; myxa is Greek for mucus) are a family of RNA viruses that includes five genera: Influenzavirus A, Influenzavirus B, Influenzavirus C, Thogotovirus and Isavirus. The first three genera contain viruses that cause influenza in vertebrates, including birds (see also avian influenza), humans, and other mammals. Isaviruses infect salmon; thogotoviruses infect vertebrates and invertebrates, such as mosquitoes and sea lice.

The three genera of Influenzavirus, which are identified by antigenic differences in their nucleoprotein and matrix protein infect vertebrates as follows:

• Influenzavirus A cause of all flu pandemics and infect humans, other mammals and birds,
• Influenzavirus B infect humans and seals, and
• Influenzavirus C infect humans and pigs.

Morphology

The virions have envelopes and occur in pleomorphic and filamentous forms. In general the virus's morphology is spherical with particles 50 to 120 nm in diameter, or filamentous virions 20 nm in diameter and 200 to 300 (-3000) nm long. There are some 500 distinct spike-like surface projections of the envelope each projecting 10 to 14 nm from the surface with some types (i.e. hemagglutinin esterase (HEF)) densely dispersed over the surface, and with others (i.e. hemagglutinin (HA)) spaced widely apart.

The major glycoprotein (HA) is interposed irregularly by clusters of neuraminidase (NA), with a ratio of HA to NA of about 4-5 to 1.

Lipoprotein membranes enclose the nucleocapsids; nucleoproteins of different size classes with a loop at each end; the arrangement within the virion is uncertain. The nucleocapsids are filamentous and fall in the range of 50 to 130 nm long and 9 to 15 nm in diameter. They have a helical symmetry.

Nucleic Acid

Viruses of this family contain 7 to 8 segments of linear negative-sense single stranded RNA.

The total genome length is 12000-15000 nucleotides (nt). The largest segment 2300-2500 nt; of second largest 2300-2500 nt; of third 2200-2300 nt; of fourth 1700-1800 nt; of fifth 1500-1600 nt; of sixth 1400-1500 nt; of seventh 1000-1100 nt; of eighth 800-900 nt. Genome sequence has terminal repeated sequences; repeated at both ends. Terminal repeats at the 5'-end 12-13 nucleotides long. Nucleotide sequences of 3'-terminus identical; the same in genera of same family; most on RNA (segments), or on all RNA species. Terminal repeats at the 3'-end 9-11 nucleotides long. Encapsidated nucleic acid is solely genomic. Each virion may contain defective interfering copies.