There will be blood: The First Plague of Egypt

The first of the famous ten plagues of Egypt was the plague of blood.

Here's the story from Exodus 7.

God told Moses to tell Aaron to smite all the water in Egypt with his rod (the one that he previously turned into a serpent and then back into a rod in Exodus 7:9-12), which will change the water into blood.

And the LORD spake unto Moses, Say unto Aaron, Take thy rod, and stretch out thine hand upon the waters of Egypt, upon their streams, upon their rivers, and upon their ponds, and upon all their pools of water, that they may become blood; and that there may be blood throughout all the land of Egypt; both in vessels of wood, and in vessels of stone. And Moses and Aaron did so, as the LORD commanded. Exodus 7:19-20

And it worked as planned. The fish died, the river stank, and the Egyptians had no water to drink.

And the fish that was in the river died; and the river stank, and the Egyptians could not drink of the water of the river; and there was blood throughout all the land of Egypt. 7:21

For seven days, apparently.

And seven days were fulfilled, after that the LORD had smitten the river. 7:25

Which must have killed some people, since, even under the most favorable conditions, a person can't survive for more than a few days without water.

But how many?

The Bible doesn't say, so I guessed 10,000.

The Friendly Atheist - How many people did God kill in the Bible?

The first plague of Egypt