Today , it is mostly referred to as Hansen 's disease , named after the person who discovered the bacterium , Gerhard Armauer Hansen . It is caused by a bacterium , Mycobacterium leprae .
Mycobacterium leprae , the causative agent of leprosy , was discovered by G. H. Armauer Hansen in Norway in 1873 , making it the first bacterium to be identified as causing disease in humans .