Historical diagnosis is a problematic, challenging, but nevertheless fascinating field – especially in the period of Middle Ages. The depiction of sickness and disease in the contemporary source material is often intermittent or follows stereotype description-patterns that do not allow an accurate identification. The chroniclers furthermore tend to moralize illness by interpreting it as a sign for impurity or sin. In most cases the retrospective analysis of medieval disease patterns can therefore only evolve possible settings and estimate their plausibility. The following treatise is a description of the „Aegritudo Arnaldia“ – the infectious disease Richard the Lion Heart and Philipp II. August of France came down with during the siege of Acre in June 1191. The symptoms handed down in both English and French sources draw a mysterious clinical picture which is characterized by high fever, ague and the loss of hair, nails and skin.