The Passover/Exodus story in Exodus 7-11 tells of ten plagues that befell Egypt. But there are shorter, poetic accounts of the plagues in Psalm 78:41 ff. and Psalm 105:27 ff. Actually, since the word plague has come to mean a viral outbreak of disease, another word or phrase might clarify that hail and locusts are not plagues in that sense. The Book of Revelation builds on the Exodus story and uses the metaphor of “bowls of wrath”. Psalm 78:43 calls them signs and wonders.
Exodus has 10 of them. The two Psalm passages have fewer.
The story in Exodus clearly has a theological meaning over against Egyptian magic and religion. This is true of both the Elohist and Priestly sources. P intensifies the polemic. It also adds the human plague of boils to the animal plagues, because apparently, P likes to attribute pestilence to the wrath of God. But to the whole Exodus account the words of Numbers 33:4 apply “YHWH executed judgment on their gods.”
I do not know if there is a historical basis for the stories. In Egyptian literature, the breakdown of social order is sometimes spoken of in terms of weather and disease. Maat is the Egyptian concept of order and balance in nature and society. As a god, it was the Pharaoh’s job to keep Maat in the land. Storms, crop failures, and plagues threatened Maat. Especially, total eclipses must have had a very negative meaning given Egyptians mythology about the sun. (On this Good Friday we remember that the sun turning to darkness portends bad and momentous events. In Christian churches we may douse the candles and shroud everything in black until Sunday).
About the best I can do about the plagues is bring some informed speculation. I believe that the Psalm 78 is the most primitive account.
The 78th Psalm is one of the Psalms of Asaph, which seem to have to have developed in northern Israel around the time of the prophet Hosea. (I dealt with this in a series of posts on this blog beginning way back in June of 2011.) But the traditions behind the Asaph Psalms may be much older.
Here are the 78th Psalms sign/plagues:
The rivers turn to blood
Swarms of flies
Death of the firstborn
However, this is a poetic account, not a prose account with a discrete section on each sign/plague, like in Exodus.
It would be better to read Psalm 78 as an account of what happened to the Egyptians as a result of God’s action. So, first they lost the Nile, their water supply. Then they lost their personal comfort to swarms of annoying critters. Then they lost their crops and livestock. Finally, they lost the lives of their posterity.
The psalm does not say anything about boils or darkness.
Psalm 78 may be closer to the poetic original. Exodus combines old sources to give us a developed theological account. And Psalm 105 is a late summary in song.
But here is something than may give us more: according to Psalm 78:43, these things happened in the field or region of Zoan (see Psalm 78:12 as well). Zoan was the Hebrew name for Avaris (Greek), the old Hyksos capital. The story in Exodus is set in or near the new Egyptian capital of Pi-Ramses (Exodus 1:11), which did not exist until the 12th century.
We used to think the city of Tanis had been built over the Hyksos capital. But we have now found the old city at Tell-el-Daba. See here.
Given that I think there were several exodus-like events, some of them with Semitic people departing from the area of Avaris/Zoan, let me ask a couple of tentative questions.
Might Psalm 78 reflect the origin of the plague motif in one of these departures close to Hyksos times?
And, given the northern-Israel connection of the Psalms of Asaph, might this go back to a memory of when the ancestors of the Joseph tribes left Egypt?