The plague of blood was followed by the plagues of frogs and lice.
The Purpose of Culture
וַיֹּאמֶר ה' אֶל מֹשֶׁה אֱמֹר אֶל אַהֲרֹן נְטֵה אֶת מַטְּךָ וְהַךְ אֶת עֲפַר הָאָרֶץ וְהָיָה לְכִנִּם וגו': (שמות ח:יב)
G‑d instructed Moses to say to Aaron, “Raise your staff and strike the dust of the earth, and it will turn into lice.” Exodus 8:12

The louse is a parasite; it lives off animals and people without contributing anything to their lives. It is therefore a metaphor for evil, since evil thrives by sucking the life force out of holiness rather than by its own merits.

Just as a louse can attach itself to a person only if his hygiene is lax, evil can only thrive when we allow our Divine consciousness to lapse, falling either into misdeeds or the apathy toward holiness that leaves us vulnerable to the enticements of materialism.

By infesting the Egyptians with lice, G‑d was showing them what their indifference to Divinity made them into: “parasites.” All their achievements in literature, art, architecture, science, and so on, served only to inflate their egos and enhance their material lives. As such, they were draining vitality from the forces of holiness in the world rather than aiding holiness.1