Original: 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.
Simplified: 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.