After becoming assistant to Andrew Sellards, he started 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.