Some viruses do a establish persistent infection that can last the lifetime of the host animal. To establish and maintain a persistent infection, the virus must avoid elimination by the immune system. Is the barrier between blood and cerebrospinal fluid. Example of viruses that undergo antigenic variations are influenza viruses and lentiviruses.
Antigenic variation is caused mostly by point mutations but also by recombination.
A 3 year study of Visna virus infection in sheep. Sheep were inoculated with plaque purified visna virus and assayed for neutralizing antibodies in serum. Serum from sheep early in the infection neutralized the parental virus strain used for inoculation but the serum did NOT neutralize later viral isolates. Antibodies obtained at the end of the study neutralized the parental strains and variant strains. Antigenic variation often results when viruses interact with the immune system, and it is an important mechanism of virus evolution and survival.