Who is credited with the discovery of the bacterium that causes leprosy?

Gerhard Armauer Hansen (July 29, 1841 – February 12, 1912) was a Norwegian physician known for his discovery of Mycobacterium leprae, the bacterium that causes leprosy. While many physicians and scientists believed leprosy to be hereditary, Hansen was convinced that a bacterium carried the disease from person to person. This was a daring speculation at a time when the concept of contagion was still poorly understood, and no one had shown that bacteria could cause human diseases.

At the age of 32, Hansen examined leprous skin nodules and discovered the rod-like bodies inside cells that looked like bacteria; they were not present in all cells, but in most of them. He believed these bodies to be the causative agent of leprosy and thereby became the first to suggest that microorganisms might cause a human disease. Hansen also conducted a study among emigrants to the United States who had leprosy. He observed that none of the subjects’ descendants developed leprosy; in combination with his discovery of the leprosy bacillus, this allowed him to establish that leprosy is a contagious disease.

Hansen's research helped to establish fundamental principles in immunology, bacteriological medicine, and public health policy.In his honor, leprosy is also referred to as Hansen’s disease.