Which Norwegian physician identified the leprosy bacterium Mycobacterium leprae in 1873?

Gerhard Armauer Hansen identified the leprosy bacterium Mycobacterium leprae in 1873. His observation provided an early crucial link between a chronic human disease and a specific microscopic organism.

Hansen worked in Bergen, Norway, where leprosy was often regarded as hereditary or associated with miasma. Studying affected tissue with improved microscopy, he observed rod-shaped bodies in samples from people with leprosy and argued that the disease was communicable and had a biological cause.

His discovery was initially incomplete by later standards: Hansen did not culture the organism or prove its infectiousness experimentally. In 1879, German bacteriologist Albert Neisser received tissue samples from Hansen and developed a staining method that made the bacillus easier to see. Neisser published his findings in 1880, creating a dispute over credit, but Hansen is conventionally recognized as the discoverer.

The bacterium is now called Mycobacterium leprae, and the disease is also known as Hansen’s disease. A related species, Mycobacterium lepromatosis, was identified much later, but it does not replace Hansen’s canonical answer for the 1873 discovery.