According to medieval thinking, the cause of the Black Plague had many sources. It could be seen as a medical event, astrological misfortune, or a sign of God's wrath. All these interpretations were offered at the same time and without contradiction. In the Decameron, Boccaccio states some possibilities: "Some say that [the plague] descended upon the human race through the influence of the heavenly bodies, others that it was a punishment signifying God's righteous anger at our iniquitous way of life. But whatever its cause, it had originated some years earlier in the East, where it had claimed countless lives before it unhappily spread westward, growing in strength as it swept relentlessly on from one place to the next" (McWilliam translation).
The most famous treatise on the causes of the plague, the Paris Consilium, was written by forty-nine medical masters at the University of Paris in October 1348 at the request of King Philip VI of France. It stated that the ultimate cause of the plague would never be known - that the truth was beyond human grasp. It did, however, give several possibilities:
The celestial cause was the result of the conjunction of Saturn, Jupiter, and Mars, under the moist sign of Aquarius, that took place in 1345, following both solar and lunar eclipses. The Paris Consilium cited Aristotle's notion that the conjunction of Saturn and Jupiter would bring disaster. According to Albert the Great, the conjunction of Jupiter and Mars would bring plague. Jupiter, the sanguine planet, was hot and wet - the two qualities that led to rotting or putrefaction which in turn led to plague.
The terrestrial cause was air poisoned from noxious gases released during earthquakes. Further unfortunate conjunctions of constellations brought on thunder, rain, and wet south winds that dispersed the poisonous vapors caused by carcasses rotting in swamps. When the poisoned air entered the body, it went to the heart - considered, in medieval times, the organ of respiration - and then contaminated the body's vital spirit and caused its organs to rot.
Getz observes that the presence of ideas taken from the Hippocratic text Epidemics - which stressed the importance of astrology in medical practice - is unmistakable. Aristotle's Meteorology, also influential, discussed weather and other atmospheric phenomena, such as comets and meteors, earthquakes, and especially putrefaction - the process regarded by medieval thinkers as the nature of illness, especially fevers. Also used by the Parisian Doctors was The Canon of the Persian physician Avicenna, which describes the nature of pestilential fever.
(B.C., ed: D.S.) Campbell, Anne. The Black Death and Men of Learning. New York: Columbia University Press, 1931; Getz, Faye Marie. "Black Death and the Silver Lining; Meaning, Continuity, and Revolutionary Change in Histories of Medieval Plague," Journal of the History of Biology 24.2 pp. 265-289; McWilliam, G. H. Giovanni Boccaccio The Decameron. London: Penguin Books, 1972.