infections of the brain caused by arthropod-borne viruses primarily from the families togaviridae, flaviviridae, bunyaviridae, reoviridae and rhabdoviridae; life cycles of these viruses are characterized by zoonoses, with birds and lower mammals serving as intermediate hosts; the virus is transmitted to humans by the bite of mosquitoes or ticks; clinical manifestations include fever, headache, alterations of mentation, focal neurologic deficits, and coma.