ADVANCES IN THE DIAGNOSIS
AND TREATMENT OF
ALL OF MEDICINE has been profoundly influenced by the work of Koch, who demonstrated the bacterial origin of tuberculosis. In 1882, Koch presented a series of elegant experiments that proved the etiologic role of the tubercle bacillus in the origin and transmission of a disease. This monumental achievement made it possible to exclude other pulmonary infections, which previously had been misdiagnosed as tuberculosis, and to include those infections that belonged there but that had previously been excluded. In this way, the category was restricted to a biologically more homogeneous population. The powerful promise of this work for meaningful classification and nosology was not lost upon Kraepelin. He recognized that the protean manifestations of tuberculosis could now be understood and classified within a single disease entity conception. Kraepelin attempted to translate and apply the work of Koch to psychiatry. Obviously, it was not possible to fulfill the requirements of Koch's postulates to obtain the bacterium and have it infect a new host, in mental disorders. Kraepelin attempted, therefore, to translate the postulates into a form more appropriate for psychiatry and to create, on the basis of clinical features, entities that shared a specific etiology, presented with a consistent picture, and followed a predictable course over time. In this way, he hoped to create diagnostic categories of mental disease that would be comparable in their clinical unity to Koch's explanation of tuberculosis.
As early as 1883, Kraepelin 25 began to use the course of illness as a classificatory variable. This attempt to use the outcome of patients suffering from mental disorders as a