This is what Boccaccio wrote about the Black Plague in the year 1348.
The following excerpt is taken from Boccaccio's The Decameron. It is a detailed description of life in the middle ages, specifically the effects of the Black Death or Bubonic Plague.
I say, then, that the years of the beatific incarnation of the Son of God had reached the tale of one thousand three hundred and forty-eight when in the illustrious city of Florence, the fairest of all the cities of Italy, there made its appearance that deadly pestilence, which, whether disseminated by the influence of the celestial bodies, or sent upon us mortals by God in His just wrath by way of retribution for our iniquities, had had its origin some years before in the East, whence, after destroying an innumerable multitude of living beings, it had propagated itself without respite from place to place, and so, calamitously, had spread into the West.
In Florence, despite all that human wisdom and forethought could devise to avert it, as the cleansing of the city from many impurities by officials appointed for the purpose, the refusal of entrance to all sick folk, and the adoption of many precautions for the preservation of health; despite also humble supplications addressed to God, and often repeated both in public procession and otherwise, by the devout; towards the beginning of the spring of the said year the doleful effects of the pestilence began to be horribly apparent by symptoms that shewed as if miraculous.
Not such were they as in the East, where an issue of blood from the nose was a manifest sign of inevitable death; but in men and women alike it first betrayed itself by the emergence of certain tumours in the groin or the armpits, some of which grew as large as a common apple, others as an egg, some more, some less, which the common folk called gavoccioli. From the two said parts of the body this deadly gavocciolo soon began to propagate and spread itself in all directions indifferently; after which the form of the malady began to change, black spots or livid making their appearance in many cases on the arm or the thigh or elsewhere, now few and large, now minute and numerous. And as the gavocciolo had been and still was an infallible token of approaching death, such also were these spots on whomsoever they shewed themselves. Which maladies seemed to set entirely at naught both the art of the physician and the virtues of physic; indeed, whether it was that the disorder was of a nature to defy such treatment, or that the physicians were at fault--besides the qualified there was now a multitude both of men and of women who practised without having received the slightest tincture of medical science--and, being in ignorance of its source, failed to apply the proper remedies; in either case, not merely were those that recovered few, but almost all within three days from the appearance of the said symptoms, sooner or later, died, and in most cases without any fever or other attendant malady.
Moreover, the virulence of the pest was the greater by reason that intercourse was apt to convey it from the sick to the whole, just as fire devours things dry or greasy when they are brought close to it. Nay, the evil went yet further, for not merely by speech or association with the sick was the malady communicated to the healthy with consequent peril of common death; but any that touched the cloth of the sick or aught else that had been touched or used by them, seemed thereby to contract the disease.
So marvellous sounds that which I have now to relate, that, had not many, and I among them, observed it with their own eyes, I had hardly dared to credit it, much less to set it down in writing, though I had had it from the lips of a credible witness.
(There is more at the link above, but I think that's enough detail for now)