They focused on an analysis of why humans had been stricken with the plague based on celestial and earthly portents.
The authors guessed at possible causes including a conjunction of Saturn, Jupiter and Mars under the water sign of Aquarius, an event they claimed had taken place on 20 March 1345 after a solar and lunar eclipse.
"Evil constellations" brought rain, thunder and moist south winds that spread the poisonous vapors given off my rotting carcasses.
In the Middle Ages the heart was believed to be the organ governing the respiratory function, so they believed that this poisoned air reached the heart through breath and contaminated the spirit, causing organs to rot.
This was based on The Canon of Medicine (القانون في الطب al-Qānūn fī al-Ṭibb) by Avicenna.