Viremia is a medical condition where viruses enter the bloodstream and hence 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.
Viral replication then leads to viremia and the virus spreads to its secondary site of infection, the central nervous system (CNS).
Vaccination must be done before secondary viremia takes place for the individual to avoid brain damage or death.
Examples include the measles, in which primary viremia occurs in the epithelial lining of the respiratory tract before replicating and budding out of the cell basal layer (viral shedding), resulting in viruses budding into capillaries and blood vessels.