Arbovirus infections are viral diseases transmitted by insect vectors—that is, they are arthropod borne. Clinically significant arboviruses in the United States include western equine encephalitis virus (WEE), eastern equine encephalitis virus (EEE), St. Louis encephalitis virus (SLE), the La Crosse (LAC) strain of the California encephalitis group, Powassan virus (POW), and Colorado tick fever virus (CTF). The four encephalitis viruses are spread by mosquitoes, whereas POW and CTF are tick borne. Not surprisingly, given that transmission depends on mosquito and tick vectors, arbovirus infections are most common in the US during the late spring and summer months. The geographic distribution of these viruses (see Table) is helpful to diagnosis because their clinical manifestations are not distinctive enough in individual patients to be diagnostic.