Counsel Against the Plague

...[it] is noticed more than the others: nevertheless, the burning furnace immediately ignites thick and green wood because the supreme power of the agent disposes and forms [it] quickly. And many times the pestilential vapor, due to the force of its causes, is most strong, and you do not know when. Furthermore, he who is not disposed at one time is disposed at another.

How the plague spreads and in which persons. Chapter III.

This poisonous vapor is many times so subtle that it is not seen, and sometimes it dissolves. But it easily regenerates while the influence disposed to this continues. Furthermore, however little it may be at the beginning, [it is] much more so than sulfur when it ignites; it spreads from place to place and runs swiftly, and without comparison it is
The vital spirit is a vapor of subtle blood.

more contrary and hostile to the vital spirit of the heart, which is a vapor of subtle blood, than sulfur is to the nose. The said vapor from the infected air ignites only in bodies that are very disposed to this, but after it is ignited in these [people], just like lit sulfur, with wonderful velocity and ease it dilates from afar much more than before, especially when it is ignited in subtle humors and in the subtle air of summer; and it scatters from one body to the other and ignites most of all where it finds great similarity