a viral disease of warm-blooded animals, notably horses (horse) and sheep, characterized by inflammation of the brain and spinal cord. Named for a severe outbreak at Borna, near Leipzig, Ger., in 1894, it is transmitted by food and water contaminated by secretions of infected animals. Mortality may reach 90 percent. Some studies have claimed a link between exposure to the Borna disease virus and mental illness (mental disorder) in humans, but such a connection has not been conclusively proven.