St. Louis encephalitis virus (SLEV) is maintained in a mosquito-bird-mosquito cycle, with periodic amplification by peridomestic birds and Culex species mosquitoes. Wild birds are the primary vertebrate hosts. Birds sustain inapparent infections but develop viremias (i.e., virus in their blood) sufficient to infect the mosquito vectors. Birds that are abundant in the urban-suburban environment, such as the house sparrow, pigeon, blue jay, and robin, are principally involved. The principal vectors are Cx pipiens and Cx quinquefasciatus in the east, Cx nigripalpus in Florida, and Cx tarsalis and members of the Cx pipiens complex in western states. Humans and domestic mammals can acquire SLEV infection, but are dead-end hosts.