Gerhard Armauer Hansen (1841-1912)
Identified the cause of leprosy (see also: Hansen's disease) — a bacillus known today as mycobacterium leprae. The discovery that leprosy was caused by a microorganism was the first step in treatment of the disease.
Unfortunately, the methods of medical practice being *rather* unfair at the time, Dr. Hansen discovered the contagiousness of the disease only after having transplanted the bacteriae deliberately onto a healthy patient's eye. History has it that the poor woman even resisted this "innovative treatment", as she only had vision on one eye to begin with.
Dr. Hansen was later convicted and lost his license.