Viremia occurs when viruses enter the bloodstream and have access to the rest of the body.
Primary viremia refers to the initial spread of virus in the blood from the first site of infection.
Secondary viremia: when primary viremia has resulted in infection of additional tissues via bloodstream, in which the virus has replicated and once more entered the circulation.
Secondary viremia results in higher viral shedding and viral loads within the bloodstream due to the possibility that the virus is able to reach its natural host cell from the bloodstream and replicate more efficiently than the initial site (rabies virus).
Active viremia is caused by the replication of viruses which results in viruses being introduced into the bloodstream: measles, in which primary viremia occurs in the epithelial lining of the respiratory tract before replicating and basal layer viral shedding, resulting in viruses budding into capillaries and blood vessels.
Passive viremia is the introduction of viruses in the bloodstream without the need of active viral replication: inoculation from mosquitoes, through physical breaches or via blood transfusions.