Indberetning til det Norske mediciniske Selskab i Christiania om en med understottelse af selskabet foretaghen reise for at anstille undersogelser angaende spedalskhedens arsager, tidels udforte sammen med forstander Hartwig.

Hansen discovered the leprosy bacillus on 28 February 1873. He had been stimulated by the previous work of Danielssen and Boeck, and his own demonstration of the leprosy bacillus is one of the earliest observations of pathogenic bacteria. For an English translation of the paper see Brit. for. med. -chir. Rev., 1875, 55, 459-89.