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.