Albert Barnes Commentary 2 Kings 19:35

"And it came to pass that night, that the angel of Jehovah went forth, and smote in the camp of the Assyrians a hundred fourscore and five thousand: and when men arose early in the morning, behold, these were all dead bodies." — 2 Kings 19:35 (ASV)

The camp of the Assyrians - Which had now been moved to Pelusium, if we can trust Herodotus; or which, in any case, was at a considerable distance from Jerusalem.

When they arose early in the morning, behold ... - These words are the only trustworthy data we have for determining, to any extent, the manner of the destruction that occurred. They imply that there was no disturbance during the night, no alarm, and no awareness among the living that their comrades were dying by the thousands all around them. All purely natural causes must be rejected. God must be seen as having slain the men in their sleep without causing any disturbance, either by a pestilence or by the kind of "visitation" spoken of in English law. The closest parallel case is the destruction of the firstborn (Exodus 12:29).

The Egyptian version of this event, as recorded by Herodotus, is that during the night, an immense multitude of field mice silently and secretly spread throughout the Assyrian army. They gnawed their quivers, bows, and shield straps, rendering them useless. When morning came, the Assyrians fled hastily, and the pursuing Egyptians put a vast number of them to the sword.