Borna disease was first described over 200 years ago, in what is now Southeastern Germany, as a fatal neurologic affliction of horses and was considered a curiosity for many decades. The causative agent was unknown, and the animal species infected in nature were limited to horses and sheep. Today, as described in this volume, the host range has extended to all warm-blooded animals, the genes and proteins of the virus have been identified, and many of the mechanisms responsible for behavioral disturbances are understood. Serologic studies suggest that BDV or related agents are likely to play a role in human neuropsychiatric diseases.