Max Theiler working on yellow fever. In 1926, they disproved Hideyo Noguchi's hypothesis that yellow fever was caused by the bacterium "Leptospira icteroides." In 1928, the year after the disease was identified conclusively as a virus, they showed that the African and South American viruses are immunologically identical. (This followed Adrian Stokes' inducing yellow fever in rhesus macaques from India). In the course of this research, Theiler contracted yellow fever, but survived and developed immunity. In 1930, Theiler moved to the Rockefeller Foundation in New York, where he later became director of the Virus Laboratory. He was professor of epidemiology and public
Arnold Theiler sickness with a combination of live virus and equine antiserum. This disease is now known to be caused by a pegivirus. Theiler was the first Director of the Onderstepoort Veterinary Research Institute, outside Pretoria. This institute under his leadership carried out research on African horse sickness, sleeping sickness, malaria, East Coast fever ("Theileria parva") and tick-borne diseases such as redwater, heartwater and biliary. The University of Pretoria Faculty of Veterinary Science was established there in 1920 which enabled veterinarians to train locally for the first time. Theiler became the first dean of this faculty. He married Emma Sophie Jegge (1861–1951)