Involvement of the brain is one of the most serious consequences of a viral infection. Many virus families have the ability to invade and replicate in brain tissue, but fortunately serious brain infections are rare. Clinically neurological diseases caused by viruses can be divided into acute and chronic syndromes. The pathology may be due either to multiplication of virus in the cells of the brain or due to the immune response of the host - post infectious encephalo-myelitis.
Viruses which infect the brain may reach the central nervous system either by the blood stream or by spread along peripheral nerves. Asymptomatic infection of the brain is common. Where a virus infects the brain directly it can usually be isolated either from brain tissue or from the cerebrospinal fluid. This is not the case with the post infectious syndromes.