Exo 8:16 And the LORD said unto Moses, Say unto Aaron, Stretch out thy rod, and smite the dust of the land, that it may become lice throughout all the land of Egypt.
Exo 8:17 And they did so; for Aaron stretched out his hand with his rod, and smote the dust of the earth, and it became lice in man, and in beast; all the dust of the land became lice throughout all the land of Egypt.
I once had to help a friend remove lice, which can cause itching and swelling, from his daughter's head. It is an unpleasant process, where you use a fine tooth comb to seek and remove them. In the days of this plague, they treated the problem in two ways, either the use of oil or shaving the hair to get rid of them. (This is why you see Egyptians in pictures depicted as bald.) Lice are also a sign of being unclean.
The Pharaoh was god on earth and his appearance meant everything. Nothing less would do than fine jewels, garments, cosmetics and coiffed hair. The lice could get in the nose and ears and if they got in the eyes they could cause injury. Ra's eye was blinded by this plague by the striking of the rod. The livestock and pets where infested with lice as well. When the wind blew it would stir up clouds of them like dust clouds.
The judgment had now come upon Egypt, Pharaoh and their gods. The gods that protected the earth for the Egyptians were Geb and Nuith. Through the plague of lice G-d had made the judgment that their gods, people, land and animals where unclean, and one of the chief gods, ra, was blind and brought their god of magic to no effect.
As we read on in Exodus we see something very profound in the following verses.
Exo 8:18 And the magicians did so with their enchantments to bring forth lice, but they could not: so there were lice upon man, and upon beast.
Exo 8:19 Then the magicians said unto Pharaoh, This is the finger of God: and Pharaoh's heart was hardened, and he hearkened not unto them; as the LORD had said.
The description of the power of the finger of G-d is used in the Scriptures often. These are only a few:
He sends plagues Exodus 8:19.
He writes the law Exodus 31:18 See also Exodus 24:12; Exodus 32:16; Exodus 34:1.
He writes judgment Daniel 5:5 See also Daniel 5:24-28.
God works miracles with his finger.
He casts out devil spirits Luke 11:20.
Picture from here