Ex 9:3 Behold, the hand of the LORD is upon thy cattle which is in the field, upon the horses, upon the asses, upon the camels, upon the oxen, and upon the sheep: there shall be a very grievous murrain.
Ex 9:4 And the LORD shall sever between the cattle of Israel and the cattle of Egypt: and there shall nothing die of all that is the children's of Israel.
Ex 9:5 And the LORD appointed a set time, saying, To morrow the LORD shall do this thing in the land.
Ex 9:6 And the LORD did that thing on the morrow, and all the cattle of Egypt died:but of the cattle of the children of Israel died not one.
Ex 9:7 And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead. And the heart of Pharaoh was hardened, and he did not let the people go.
With limited knowledge of medicine when something unknown happened to an animal the Egyptians would go to the gods and make a sacrifice for a cure or seek a "wise one" or magician. This plague fell only on domestic animals. Murrainis best explainedin today's terms as anthrax, hoof and mouth, mad cow and streptococcus. Normally what would take weeks to wipe out herds of cattle and camels and sheep took hours. This brought Egypt to a stand still. Camels and donkeys were used to ride and to haul merchandise and supplies, so this was the equivalent of shutting down every truck and land shipping means known to man. Great losses of food were suffered. You cannot eat diseased animals. It would take years to replace them, even if you imported new livestock.
Picture from here