German doctors in the General Government played an important role in providing the medical rationalization for ghettoization and mass murder. Their desire to prevent the spread of disease to Germans led them to favour providing adequate health care for Poles. The same self-interest engendered their persistent advocacy of ghettoization for Jews, who were believed to be natural carriers of spotted fever. When ghetto conditions created a self-fulfilling prophecy of wide-spread disease, the doctors advocated tighter sealing of the ghettos. By late 1941, this self-induced threat to public health made the doctors receptive to a mass murder solution.