In this review data are presented which indicate a high degree of genetic stability of BDV in his natural host, the horse. Despite this high degree of sequence conservation, variation in antigenicity was found, which did not influence the pathogenic properties of the virus. In addition, the correlation between BDV-seropositivity and a variety of psychiatric and neurological disorders in humans is discussed. In diagnostically unselected psychiatric patients we found a similar distribution of psychiatric disorders in BDV seropositives compared to seronegatives. Investigations of cerebrospinal fluid revealed cases of BDV encephalitis in BDV seropositive psychiatric and neurological patients. In contrast to others, we have found no evidence for the presence of BDV-RNA or BDV in human peripheral blood leucocytes.