In a report published in 1921, Theiler 1 stated that at least seven enzootics of African horse sickness (AHS) had occurred in the Cape Province during the period from 1780 to 1918. Prior to that time, there were no horses in South Africa, and the disease was not known there or elsewhere. The disease emerged only after horses from Europe and the East Indies were imported to South Africa. It was not immediately recognized as a separate disease entity. Because it had some clinical features similar to anthrax and piroplasmosis, it was often mistaken for those diseases. Various bacteria, parasites, and even a fungus were suggested as possible causative agents of AHS. However, in 1900, M’Fadyean 2 succeeded in transmitting the disease with a bacteria-free filtrate of blood from an infected horse. These findings were confirmed by others 1 year later, 3–5 and it was generally accepted that the causative agent was a virus.