Within the nervous system the highly specialized structure and function of nerve cells renders the pathogenesis of viral infections amazingly complex. In vivo and in vitro studies reveal that viruses may display tropism for distinct types of cells such as neurons, myelin-forming cells, or astrocytes. In neurons, RNA viruses mature in the cell body and in dendrites close to synapses, from which they can spread to synaptic endings. Undefined host factors and stage of differentiation may favor defective viral assembly, which, in turn, results in persistent infections of neurons. In myelin-forming cells, lytic infection results is persistent infections of neurons. In myelin-forming cells, lytic infection results in degeneration of myelin and, consequently, in altered conduction in those axons that are ensheathed by a myelin-forming cell. In addition, breakdown of myelin may induce an autoimmune response, which then leads to further demyelination. Autoimmune demyelination may also occur when glial cells other than myelin-forming cells are infected. Astrocytes are prone to persistent infection or viral transformation.