The article examines two seemingly unconnected occurrences at the nineteenth-century north east frontier of British India. The first is the production of a pathological space via moral, social, and cultural codes enacted by medical topographies on the region since the first Anglo-Burmese war (1824–1826) and the subsequent rise of disease thinking. The second is the ambivalence in disease thinking that is brought to fore through the mysterious malady called kala azar (visceral leishmaniasis), which was geographically designated as Assam fever. This article contends that the geographical designation of kala azar as Assam fever is not just coincidental or a nosological confusion of the late nineteenth century but rather has its origin in the preceding pathological carving of space at the frontier. Further, it explores the troubled ontology between research on malaria and kala azar investigations to show that the old codes enacted by medical topographies hinged upon the era of laboratory medicine.