Most alphaviruses and flaviviruses survive in nature by replicating alternately in a vertebrate host and a hematophagous arthropod (mosquitoes or, for some flaviviruses, ticks). Arthropod vectors acquire the viral infection by biting a viremic host, and after an extrinsic incubation period during which the virus replicates in the vector's tissues, they transmit virus through salivary secretions to another vertebrate host