An alphavirus belongs to the group IV Togaviridae family of viruses, according to the system of classification based on viral genome composition introduced by David Baltimore in 1971. Alphaviruses, like all other group IV viruses, have a positive sense, single-stranded RNA genome. There are thirty alphaviruses able to infect various vertebrates such as humans, rodents, fish, birds, and larger mammals such as horses as well as invertebrates. Transmission between species and individuals occurs mainly via mosquitoes making the alphaviruses a contributor to the collection of Arboviruses – or Arthropod-Borne Viruses. Alphavirus particles are enveloped, have a 70 nm diameter, tend to be spherical (although slightly pleomorphic), and have a 40 nm isometric nucleocapsid.