Explanation and summary
The sixth plague of the 10 plagues that the God of the Israelites sent as judgment upon Egypt, through Moses, was boils, as recorded in the book of Exodus. In this plague, boils break out on all people in Egypt. The plague insulted any god represented by an animal, like Hathor.
In the plague of boils ashes were used, probably in the same way and to the same end as the clay was used in opening the eyes of the blind man (John 9:6), i.e. to attract attention and to fasten the mind of the observer upon what the Lord was doing. This plague in the order of its coming, immediately after the murrain, and in the description given of it and in the significant warning of the "pestilence" yet to come (Exodus 9:15), appears most likely to have been pestis minor, the milder form of bubonic plague.
Virulent rinder-pest among cattle in the East is regarded as the precursor of plague among men and is believed to be of the same nature. It may well be, as has been thought by some, that the great aversion of the ancient Egyptians to the contamination of the soil by decaying animals was from the danger thereby of starting an epidemic of plague among men (Dr. Merrins, Biblical Sacra , 1908, 422-23).
Biblical Text (KJV)
8 And the Lord said unto Moses and unto Aaron, Take to you handfuls of ashes of the furnace, and let Moses sprinkle it toward the heaven in the sight of Pharaoh.
9 And it shall become small dust in all the land of Egypt, and shall be a boil breaking forth with blains upon man, and upon beast, throughout all the land of Egypt.
10 And they took ashes of the furnace, and stood before Pharaoh; and Moses sprinkled it up toward heaven; and it became a boil breaking forth with blains upon man, and upon beast.
11 And the magicians could not stand before Moses because of the boils; for the boil was upon the magicians, and upon all the Egyptians.
12 And the Lord hardened the heart of Pharaoh, and he hearkened not unto them; as the Lord had spoken unto Moses.
International Standard Bible Encyclopedia, which is in the public domain (with minor edits).