Adam Clarke on Exodus 9:9

Shall be a boil - שחין shechin. This word is generally expounded, an inflammatory swelling, a burning boil; one of the most poignant afflictions, not immediately mortal, that can well affect the surface of the human body. If a single boil on any part of the body throws the whole system into a fever, what anguish must a multitude of them on the body at the same time occasion!

Breaking forth with blains - אבעבעת ababuoth, supposed to come from בעה baah, to swell, bulge out; any inflammatory swelling, node, or pustule, in any part of the body, but more especially in the more glandular parts, the neck, arm-pits, groin, etc. The Septuagint translate it thus: Και εσται ἑλκη φλυκτιδες αναζεουσαι· And it shalt be an ulcer with burning pustules. It seems to have been a disorder of an uncommon kind, and hence it is called by way of distinction, the botch of Egypt, Deu 28:27, perhaps never known before in that or any other country. Orosius says that in the sixth plague "all the people were blistered, that the blisters burst with tormenting pain, and that worms issued out of them." Alfred's Oros., lib. i., c. vii.