Albert Barnes Commentary Exodus 9:9

"And it shall become small dust over 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." — Exodus 9:9 (ASV)

A boil probably means a burning tumor or carbuncle breaking out in pustulous ulcers. The miracle consisted in the severity of the plague and its direct connection with the act of Moses.