http://www.fordham.edu/halsall/source/decameronintro.html
Boccaccio: THE DECAMERON , “INTRODUCTION”
Thirteen hundred and forty-eight years had passed since the fruitful Incarnation of the Son of God, when there came into the noble city of Florence, the most beautiful of all Italian cities, a deadly pestilence, which, either because of the operations of the heavenly bodies, or because of the just wrath of God...
Just guessing, but it might depend upon the origin of the reports, Earth was the centre of the Universe...and ‘rocks’ didn’t fall from the sky. Boccaccio was very brave to allude to ‘the operations of the heavenly bodies’ methinks.
Types of cometary forms, illustrations from Johannes Hevelius' Cometographia (Danzig, 1668) Click on image for larger view. Image credit: NASA/JPL
Woodcut showing destructive influence of a fourth century comet from Stanilaus Lubienietski's Theatrum Cometicum (Amsterdam, 1668). Click on image for larger view. Image credit: NASA/JPL