During the 19th century leprosy was a serious health problem in Norway, especially in some western, rural districts. In 1856 it was decided that all leprous patients should be examined by the local doctor (District Health Officer), and registered in a national leprosy register. The patients' family relationships received special attention. Some patients tried to avoid registration, fearing that the data might be misused. After Armauer Hansen (1841-1912) discovered in 1873 that leprosy was an infectious disease, isolation of leprous patients was enforced. In 1884 Thomas Collett (1835-1898), the local doctor in a rural district of western Norway, carried out a survey of all leprous patients registered in his district, a total of 164 patients. The data from his survey provide convincing support for the view that hereditary factors play an important role in the development of the disease. Modern research has confirmed that an important gene controls the susceptibility to leprosy.