Vertebrates are one of three components that are necessary for transmission of an arthropod-borne virus (arbovirus); the three essential parts are the vertebrate host, the virus, and the arthropod vector. The principal role of a vertebrate host is to become viremic; i.e., to circulate virus in its peripheral blood. If the viremia is sufficiently high, the virus will infect blood-feeding vectors. Most arbovirus studies that addressed vertebrate host ecology focused on the analysis of blood for virus or antibody, the distribution and abundance of potential hosts, vector-host interactions, or experimental infections. After 85 years of arbovirus research, there is still much to learn concerning how vertebrates affect arbovirus transmission and, conversely, how vectors and viruses affect vertebrates. All vertebrates do not contribute equally to virus transmission, and the role of a given species, group of species, or individuals within a species in a transmission cycle varies for different viruses and geographic locations.