The plagues that God sent upon the Egyptians corresponded to the deeds they had perpetrated against the children of Israel. Because they forced the Israelites to draw water for them, and also hindered them from the use of the ritual baths, He changed their water into blood.
Because they had said to the Israelites, "Go and catch fish for us," He brought frogs up against them, making them to swarm in their kneading-troughs and their bed-chambers and hop around croaking in their entrails. It was the severest of all the ten plagues.