Mycobacterium leprae
Leprosy was regarded as largely hereditary or otherwise miasmic in origin. Hansen concluded on the basis of epidemiological studies that leprosy was a specific disease with a specific cause. In 1870–71 Hansen travelled to Bonn and Vienna to gain the training necessary for him to prove his hypothesis. In 1873, he announced the discovery of "Mycobacterium leprae" in the tissues of all sufferers, although he did not identify them as bacteria, and received little support. The discovery was made with a "new and better" microscope.