A variety of viruses can cause diseases of the central nervous system by two different mechanisms. Cytolytic viruses exert direct effects on cells of the nervous system that result in death and cell loss. Noncytolytic viruses do not directly damage cells, but might harm cell functions in the long run. However, more common is the reaction of the immune system towards viral antigens of those noncytolytic viruses which ultimately might lead to an immunopathological disease. Several peculiarities of the central nervous system with regard to an immune reaction within this organ are discussed in the context of viral infections.