What was the cause of Charlemagne's death?

Charlemagne (2 April 748 – 28 January 814), numbered Charles I, was King of the Franks from 768, King of the Lombards from 774, and Emperor of the Romans from 800. During the Early Middle Ages, he united the majority of western and central Europe. He was the first recognised emperor to rule from western Europe since the fall of the Western Roman Empire three centuries earlier. The expanded Frankish state that Charlemagne founded is called the Carolingian Empire.

In 813, Charlemagne called Louis the Pious, king of Aquitaine, his only surviving legitimate son, to his court. There Charlemagne crowned his son as co-emperor and sent him back to Aquitaine. He then spent the autumn hunting before returning to Aachen on 1 November. In January, he fell ill with pleurisy.

In deep depression (mostly because many of his plans were not yet realised), he took to his bed on 21 January and as Einhard tells it: "He died January twenty-eighth, the seventh day from the time that he took to his bed, at nine o'clock in the morning, after partaking of the Holy Communion, in the seventy-second year of his age and the forty-seventh of his reign."