What bacterium causes leprosy?

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Leprosy is caused by the bacterium Mycobacterium leprae.12

This organism is a rod-shaped bacterium and a relative of M. tuberculosis, the pathogen responsible for tuberculosis. It was first identified in 1873 by the Norwegian physician Gerhard Armauer Hansen, which is why the condition is also known as Hansen’s disease.21

The bacterium is unique in that it has never been successfully grown in tissue culture, which has historically made it difficult to study. While it is transmitted through prolonged close contact between humans, research also suggests that in some regions, such as the southern United States, it may be a zoonotic disease linked to contact with armadillos.2