Boccaccio records the outbreak of the plague in The Decameron, the first day.
"In the year 1348 after the fruitful incarnation of the Son of God, that most beautiful of Italian cities, noble Florence, was attacked by deadly plague. It started in the East either through the influence of the heavenly bodies or because Godís just anger with our wicked deeds sent it as a punishment to mortal men; and in a few years killed an innumerable quantity of people. Ceaselessly passing from place to place, it extended its miserable length over the West. Against this plague all human wisdom and foresight were vain. Orders had been given to cleanse the city of filth, the entry of any sick person was forbidden, much advice was given for keeping healthy; at the same time humble supplications were made to God by pious persons in processions and otherwise. And yet, in the beginning of the spring of the year mentioned, its horrible results began to appear, and in a miraculous manner. The symptoms were not the same in the East, where a gush of blood from the nose was the plain sign of inevitable death; but it began both in men and women with certain swellings in the groin or under the armpit They grew to the size of small apple or an egg, more or less, and were vulgarly called tumors. In a short space of time these tumors spread from the two parts named all over the body. Soon after this the symptoms changed and black or purple spots appeared on the arms or thighs or any other part of the body, sometimes a few large ones, sometimes many little ones. These spots were a certain sign of death, just as the original tumor had been and still remained."
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Wilkie, Brian, ed., The Decameron in Literature of the Western World. (New York 1992), p. 1830.