The Crime of Doctor Hallet (1938)

Working in the jungle, a missionary doctor and his assistant try to find a cure for the dreaded red fever. One of the doctors, believing his work in the field to be useless, offers himself to his rival as a human guinea pig...and dies in the process. The other doctor assumes the dead man's identity to complete the dead man's unfinished work and does so. This leads to a great many complications involving one doctor's wife and the other doctor's girlfriend.