What does Exodus 8:21 mean?

The first two plagues against Egypt came with warnings (Exodus 7:17; 8:2). The third records no prior notice, but it implies an obvious act of God (Exodus 8:19). The fourth plague once again comes with an ultimatum to Pharaoh: release the Hebrew slaves (Exodus 1:11–14; 5:1–4; 8:20). Else, the Egyptians will be swarmed with flies. The threat is descriptive. The flies will be everywhere, including in Pharaoh's own home and even covering the ground. However, the people of Israel will be spared (Exodus 8:22), reinforcing the miraculous nature of the plague. This detail also disproves any power the Egyptian idols might have claimed over nature.

Flies are present nearly everywhere on earth. They are drawn to waste, food, carrion, and almost anything else that can be eaten. Some give irritating bites. They are large enough to be a major nuisance, not to mention carrying disease. A major infestation of flies would create a dirty, distressing environment. Having so many flies that they cover the ground would be a nightmare. Unlike prior plagues, there is no indication that Pharaoh's magicians even tried to duplicate the effect. They may have known, by now, that there was no point. Appeals to Egyptian idols like Wadjet had already failed.

This plague also comes after—or possibly even during—prior catastrophes. The Nile was turned into blood for seven days (Exodus 7:21, 25). Dead frogs were piled in reeking heaps (Exodus 8:14). Tiny insects were everywhere (Exodus 8:17). God may have caused the explosion of flies using these very factors. Or, they may have been an entirely new problem. The next plague, on livestock (Exodus 9:3), might also have been sparked by a major fly infestation.