Emil Kraepelin's habilitation and his thesis: a pioneer work for modern systematic reviews, psychoimmunological research and categories of psychiatric diseases.

OBJECTIVES: Although Kraepelin and his oeuvre have been in scientific focus over the past decades, main aspects have still been neglected. Thus the exact circumstances under which and on what topic Kraepelin wrote his habilitation thesis and qualified as university lecturer (in Germany the prerequisite to be appointed as professor) are still widely unknown.

METHODS: This study reconstructs his habilitation at the Medical Faculty of Leipzig University in 1882.

RESULTS: The study reveals the difficulties he had to habilitate on a topic from Wilhelm Wundt's experimental psychology and the opposition he faced from Paul Flechsig. Yet Kraepelin succeeded, mainly due to a positive review by neurologist Wilhelm Erb on his study "On the Influence of Acute Diseases on the Development of Mental Illnesses" (1881/82).

CONCLUSIONS: This work must be regarded as his actual habilitation thesis. It provides an update of organic psychiatric disorders following acute inflammatory diseases and a meta-analysis on the basis of raw data. In addition it discusses possibilities to categorize and understand the pathophysiological mechanism of these disorders and to classify them into those appearing when the fever rises and those occurring when it falls, which has a high impact from a very modern psychoimmunological viewpoint.