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"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. Hansen observed a number of nonrefractile small rods in unstained tissue sections. The rods were not soluble in potassium lye, and they were acid- and alcohol-fast. In 1879, he stained these organisms with Ziehl's method and noted the similarities with Koch's bacillus ("Mycobacterium tuberculosis"). There were three significant differences between these organisms: