The Bible said: By the sweat of your brow you would earn your bread until you returned to the ground, for from it you were taken; dust you were, and to dust you would return.
When the Israelites wanted to leave Egypt, Pharaoh refused to allow it, so Aaron and Moses each employed miraculous arts.
Aaron stretched out his staff and struck the dust of the earth, turning it into lice throughout all Egypt.
Moses, meanwhile, took several handfuls of furnace ash and scattered it toward the sky before Pharaoh. The ash became dust across all Egypt, clinging to people and livestock and turning into festering sores. The magicians could not stand before Moses, for they and all the people of Egypt were covered in sores.
This skin disease spread across the country in an instant. It was not as deadly as the Black Death, but it created a terrifying visual effect.
Slaves were Egyptian property, and the Hebrews were especially skilled at herding. Of course the egyptians would not let them go. It was not until darkness fell and the firstborn children died, from princes to prisoners' sons, that the egyptians agreed to let the Hebrews leave.
But no sooner had they left than the army assembled behind them. What had once been a dispute over property had now become a blood feud. There was no way the people of Egypt would not seek revenge.
Dust had originally been soil. When it became too dry, the wind lifted it and turned it into dust. It carried huge amounts of bacteria and insect eggs, things that were usually difficult to see unless they were under sunlight, where they looked like specks of gold dust.
To modern people, it was common knowledge that the plague spread through fleas carried by rats.