Virus particles can remain localized or spread to other tissues. There is local spread of the infection when newly released virus infects adjacent cells, but it is limited by physical constraints of the tissue and innate antiviral defenses. One important mechanism for viruses to avoid local antiviral defenses and become systemic is by directional release of virus particles from polarized cells at the mucosal surface. Spread through the bloodstream is called the hematogenous spread. Release of virus particles into blood after initial replication at the site of entry is called primary viremia.
Viremia is the presence of infectious virus particles in the blood.
The virions may be free in the blood, or contained within infected cells such as lymphocytes. The amount of virus released in blood during primary viremia is low. Active viremia produced by virus replication at the site of entry Passive viremia virus particles are introduced without virus replication e.g direct injection of virus into veins. Generally, viremia are short-lived, except in few infections such as hepatitis B and C, LCMV which can last for years. Either the virus attaches onto endothelial cells and migrates through these fenestration to invade underlying tissues, or the virus is carried through by infected monocytes or lymphocytes in a process called diapedesis .