Borna disease is an endemic progressive encephalomyelitis of horses and sheep prevalent in central Europe. A wide variety of animal species, ranging from chickens to primates can be infected experimentally with the causative virus, which is only poorly characterized. Furthermore, BD virus-specific antibodies have been detected in sera and cerebrospinal fluids of psychiatric patients. Our studies on the pathogenesis of BD have shown that-at least in rats-the disease is not caused by the infecting virus itself, but by a virus-induced immunopathological reaction. Thus, after intracerebral infection immunoincompetent rats do not get the disease despite persistent virus replication in cells of the central nervous system. However, after adoptive transfer of immune cells from diseased rats, immunoincompetent rats exhibit full-blown BD. Recently, we have been successful in establishing a virus-specific T cell line of the helper/inducer phenotype (CD4+). This T cell was shown to play an important role in the pathogenesis of BD, suggesting that the disease is caused by a delayed type hypersensitivity reaction.