For 23 years, Charles V “The Wise” of France suffered from a mysterious fistula on his left arm that continuously drained pus. For all this time, he believed that as soon as the fistula ceased to weep, he would have a mere 15 days before death followed. His death in 1380, at the young age of 42, seemingly proved this assumption correct. This paper explores the possible explanations from arsenic poisoning at the hands of his longtime nemesis Charles II, as many of his contemporaries believed, to an undiagnosed case of hidradenitis suppurativa, to an underlying tuberculosis infection, to the possibility that his condition was entirely self-inflicted. While it is impossible to determine a definitive cause, it is highly unlikely that Charles II "The Bad" of Navarre had anything to do with his rival's strange condition.