A boil - Means probably a burning tumor or carbuncle breaking out in pustulous ulcers. The miracle consisting in the severity of the plague and its direct connection with the act of Moses.
Related Commentaries of Exodus 9:9
Exodus 9:9
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.