Borna disease, also known as sad horse disease, is an infectious neurological syndrome of warm-blooded animals, caused by Borna disease viruses 1 and 2 (BoDV-1/2). BoDV-1/2 are neurotropic viruses of the species Mammalian 1 orthobornavirus, and members of the Bornaviridae family within the Mononegavirales order.