Comentario sobre Exodo 9:9

לשחין פרח אבעבעת A BOIL BREAKING FORTH INTO BLAINS — Render it as the Targum does: a boil growing blains, — the idea is, that through it (the boil) blains burst out on them (on the people).

AND IT SHALL BECOME SMALL DUST OVER ALL THE LAND OF EGYPT. According to the opinion of our Rabbis,²⁰⁴Shemoth Rabbah 11:6. [the small quantity of] soot [in the hands of Moses] became the dust which settled over the whole land of Egypt, and that dust, coming upon man and upon beast, caused them to break forth with boils and blains throughout all the land of Egypt, since it was a burning hot dust. Perhaps the wind caused the dust to enter the homes as well, and there was thus no escape from it. This is a correct [conjecture]. Many times during a drought, the fall of the dew is accompanied by a sort of dust, and it is furthermore written, The Eternal will make the rain of thy land powder and dust.²⁰⁵Deuteronomy 28:24.
It is also possible to say, in line with the plain meaning of Scripture, that the purport of the expression, And it shall become small dust, is that the dust which will be produced in that place from the soot will bring the boils over all the land of Egypt, as He infected the air to do so, it being a decree of the Supreme One.²⁰⁶According to this interpretation, the miracle entailed was thus greater than the one in consonance with the first interpretation, which had the dust throughout the land of Egypt causing the boils and the blains. According to the second interpretation, the soot of the furnace became dust only over the place where the miracle was wrought, which in turn caused the whole atmosphere over Egypt to effect the boils.

פורח, chemically active, producing spontaneous growth of bacteria.