Sh'mot 9:1-7 Then the L-RD said to Moshe, "Go to Pharaoh and speak to him, 'Thus says the L-RD, the G-d of the Hebrews," Let My people go, that they may serve Me. For if you refuse to let them go, and continue to hold them, look, the hand of the L-RD will come with a very severe pestilence on your livestock which are in the field, on the horses, on the donkeys, on the camels, on the herds, and on the flocks. But the L-RD will make a distinction between the livestock of Israel and the livestock of Egypt, so that nothing will die of all that belongs to the sons of Israel."' " And the L-RD set a definite time, saying, "Tomorrow the L-RD will do this thing in the land." So the L-RD did this thing on the morrow, and all the livestock of Egypt died; but of the livestock of the sons of Israel, not one died. And Pharaoh sent, and look, there was not even one of the livestock of Israel dead. But the heart of Pharaoh was hardened, and he did not let the people go.
This is the fifth plague - The plague on the livestock. Note: This plague affected more than one type of livestock. When a plague usually occurs in cattle for example, the chickens are usually not affected. In this case, the plague impacted all types of livestock. Livestock was important to the Egyptians. The bull was considered sacred and was worshipped. So was the cow. This plague was against Apis the Egyptian bull god and Hathor the Egyptian cow goddess.
In the field - This statement seems to indicate that only the animals out in the fields were affected and not the animals in the barns and stables. In the following plagues there is further indication that this is true because the other plagues affect the livestock as well.
Make a distinction - G-d protected the livestock of the Israelites. Pharaoh sent out his own investigators to determine if Israel's livestock had been spared.
Sh'mot 9:8-12 Then the L-RD said to Moshe and Aaron, "Take for yourselves handfuls of soot from a kiln, and let Moshe throw it toward the sky in the sight of Pharaoh. And it will become fine dust over all the land of Egypt, and will become boils breaking out with sores on man and beast through all the land of Egypt." So they took soot from a kiln, and stood before Pharaoh; and Moshe threw it toward the sky, and it became boils breaking out with sores on man and beast. And the magicians could not stand before Moses because of the boils, for the boils were on the magicians as well as on all the Egyptians. And the L-RD hardened Pharaoh's heart, and he did not listen to them, just as the L-RD had spoken to Moses.
This is the sixth plague - The plague of boils. It was the first plague affecting human health. It was directed against Thoth (Imhotep) the Egyptian god of medicine, intelligence and wisdom. When the Egyptians made a human sacrifice to him, the ashes from the sacrifice were cast into the air and the scattered ashes were considered to be a blessing on the worshippers of Thoth.
Moshe used the ashes from the brick kiln to initiate this plague. The brick kiln was the source of much of the Israelite hardship. Here the bricks were made that would be used by the Israelite slaves in building Pharaoh's cities. Now those ashes were being used to plague the Egyptians.
This kind of thing will happen in the future. Those that know the L-rd will have been take away into the New Jerusalem and will not see His wrath. (Rev. 3:10-13)
Revelation 16:10-11 And the fifth angel poured out his bowl upon the throne of the beast; and his kingdom became darkened; and they gnawed their tongues because of pain, and they blasphemed the G-d of heaven because of their pains and their sores; and they did not repent of their deeds. NAS
Many including Israel still harden their heart against the Father by rejecting His Son. G-d gave them over to being hardened.
Romans 11:7-10 What then? That which Israel is seeking for, it has not obtained, but those who were chosen obtained it, and the rest were HARDENED; just as it is written, "G-d gave them a spirit of stupor, Eyes to see not and ears to hear not, Down to this very day." And David says, "Let their table become a snare and a trap, and a stumbling block and a retribution to them. "Let their eyes be darkened to see not, and bend their backs forever." NAS
Sh'mot 9:13-17 Then the L-RD said to Moshe, "Rise up early in the morning and stand before Pharaoh and say to him, 'Thus says the L-RD, the G-d of the Hebrews," Let My people go, that they may serve Me. For this time I will send all My plagues on you and your servants and your people, so that you may know that there is no one like Me in all the earth. For if by now I had put forth My hand and struck you and your people with pestilence, you would then have been cut off from the earth. But, indeed, for this cause I have allowed you to remain, in order to show you My power, and in order to proclaim My name through all the earth. Still you exalt yourself against My people by not letting them go.
If by now - G-d is telling Pharaoh that He could have taken his life long ago but He kept him alive to show Pharaoh His power and greatness. The Pharaoh's were believed to be gods by the Egyptian people. It must have been an interesting scene to witness Moshe, a slave, relaying this message to a guy who thought of himself as a god.