Using Borna disease as a model, the consequences of persistent virus infections of the central nervous system, in particular of "slow virus diseases", are briefly described. Both latent and active forms of the disease have much in common with chronically progressive diseases of the central nervous system in animals and man which have until now remained unexplained. Virologically and serologically there exist simple laboratory procedures to examine the pathogenesis and immunology of the disease, in addition to suitable laboratory animals and a variety of cell cultures.