Any member of the genus Alphavirus of antigenically related togaviruses that infect a wide range of mammals, birds, and other vertebrates, are transmitted by mosquitoes (or, rarely, other arthropods), and typically cause either a syndrome of fever, rash, and arthralgia (Old World alphaviruses) or a potentially fatal encephalitis (New World alphaviruses, or equine encephalitis viruses). Also (in form Alphavirus): the genus itself.