And I will bring a sword upon you, that shall avenge the quarrel of my covenant: and when ye are gathered together within your cities, I will send the pestilence among you; and ye shall be delivered into the hand of the enemy. (Leviticus 26:25)
God had entered into covenant relation with Israel, and when they failed to keep their part of the covenant He would send the sword upon them, and also pestilence. Ezekiel repeated this threat (Eze. 5:12), which was fulfilled in the destruction of Jerusalem by Nebuchadnezzar and later by the Romans.