Flavivirus, renamed Orthoflavivirus in 2023, is a genus of positive-strand RNA viruses in the family Flaviviridae. The genus includes the West Nile virus, dengue virus, tick-borne encephalitis virus, yellow fever virus, Zika virus and several other viruses which may cause encephalitis, as well as insect-specific flaviviruses (ISFs) such as cell fusing agent virus (CFAV), Palm Creek virus (PCV), and Parramatta River virus (PaRV). While dual-host flaviviruses can infect vertebrates as well as arthropods, insect-specific flaviviruses are restricted to their competent arthropods. The means by which flaviviruses establish persistent infection in their competent vectors and cause disease in humans depends upon several virus-host interactions, including the intricate interplay between flavivirus-encoded immune antagonists and the host antiviral innate immune effector molecules.

Flaviviruses are named for the yellow fever virus; the word flavus means 'yellow' in Latin, and yellow fever in turn is named from its propensity to cause yellow jaundice in victims.

Flaviviruses share several common aspects: common size (40–65 nm), symmetry (enveloped, icosahedral nucleocapsid), nucleic acid (positive-sense, single-stranded RNA around 10,000–11,000 bases), and appearance under the electron microscope.

Most of these viruses are primarily transmitted by the bite from an infected arthropod (mosquito or tick), and hence are classified as arboviruses. Human infections with most of these arboviruses are incidental, as humans are unable to replicate the virus to high enough titers to reinfect the arthropods needed to continue the virus life-cycle – humans are then a dead end host. The exceptions to this are the yellow fever virus, dengue virus and zika virus. These three viruses still require mosquito vectors but are well-enough adapted to humans as to not necessarily depend upon animal hosts (although they continue to have important animal transmission routes, as well).

Other virus transmission routes for arboviruses include handling infected animal carcasses, blood transfusion, sex, childbirth and consumption of unpasteurised milk products. Transmission from nonhuman vertebrates to humans without an intermediate vector arthropod however mostly occurs with low probability. For example, early tests with yellow fever showed that the disease is not contagious.

The known non-arboviruses of the flavivirus family reproduce in either arthropods or vertebrates, but not both, with one odd member of the genus affecting a nematode.

Source: Wikipedia

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