Today nothing seems clearer than the distinction between infectious and genetic disorders. One is environmental; the other, inherited. But both societies and physicians have struggled with this concept. Until the mid-1800s, infectious agents were unknown, while many societies were tight-knit with inbreeding the normal consequence.
Leprosy is a classic infectious disease. In the 1850s, it had a prevalence of 25:1000 in the region around Bergen, Norway. The local experts in Bergen were Daniel Danielssen, perhaps the first leprologist, and Carl Wilhelm Boeck, a dermatologist (the uncle of Cesar Boeck, whose name is attached to sarcoidosis). They considered leprosy a hereditary dyscrasia and felt that minimizing hereditary transmission was the key to control. Their student Armauer Hansen discovered the lepra bacillus (Mycobacterium leprae), establishing leprosy as an infection.1
Burgdorf WHC, Hoenig LJ. Everyone’s Got It … It Must Be Familial. JAMA Dermatol. 2015;151(4):388. doi:10.1001/jamadermatol.2014.360