About two weeks after entering the body, the virus starts infecting immune cells themselves - specifically T cells - and that is considered the secondary viremia.
In primary viremia the virus infects a part of immune system in the liver and spleen, called the reticuloendothelial system, which is made of phagocytic cells.
Birds act as a reservoir for the virus, meaning the virus can replicate at high enough levels to cause significant viremia, or elevated viral blood counts, which allows for transmission to other uninfected mosquitoes.