According to the Bible, the first plague God visited upon Egypt targeted the Nile; as a result of this plague, the waters of the river turned into blood. This caused fish in the water to die, which created a strong unpleasant odor and also made the water undrinkable.
God turns the Nile river to blood when Moses, through his brother Aaron, strikes his staff into the water. However, the pharaoh was not convinced that Moses caused this plague to occur because members of his court were able to create the same effect. As a result, the next plague, frogs, was brought about.