What are "Dever" and "Ketev"?
Rashi (from Shocher Tov): They are names of demons 1 . The former damages at night, and the latter in the afternoon.
Radak: They are cold and heat. "Ketev" is like "Ehi Katavcha She'ol" (Hoshe'a 13:14).
Malbim: ("Dever" is a plague.) Since it said (verse 3) that Hashem will save you from Dever Havos, here it says that you need not fear Dever that goes at night.
Radak: Researchers say that demons do not exist. (The Gemara mentions demons often, and warns to be concerned for them in several situations! Perhaps Radak means that in his days, they are not found. Teshuvos v'Hanhagos (3:256) says that they are not found among us; Hagahos Mordechai (Perek ha'Motzi, 461) said so about Ru'ach Ra'ah. Salmas Chayim (YD 184) says that the Rema (YD 198:6) is lenient about a matter from which an Isur Kares could result, due to concern for Shedim, and the Poskim did not say that this no longer applies; he wavered about this. - PF)
What is the meaning of "Yashud"?
Rashi: It plunders. Radak - the Shuruk is in place of a Cholem. We find like this - "Yarun v'Some'ach" (Mishlei 29:6).
