Zornberg, Avivah Gottlieb. The Particulars of Rapture (p. 108). Knopf Doubleday Publishing Group. Kindle Edition.
The Egyptian sickness—the boils in skin and flesh—is related to the question of listening. A basic indisposition to listen is Pharaoh’s pathology. A misalignment of the borders of his body, a fantasy of total control, without entry or exit (“No slave ever escaped from Egypt”), erupts in a malignant thickening, a deadening of the body-limits that are constituted by the skin. His magicians, whose culture of witchcraft supports Pharaoh’s defiance of God, are afflicted with these miserable boils, which express a kind of “thick skin” obstinacy.

Dr. Ismar Schorsch, Parshat Va'era, originally published in 1996, posted on jtsa.edu in 1998
“To harden the heart” is a figure of speech that goes back to the book of Exodus. Associated with Pharaoh’s defiance of God’s recurring plea, “Let My people go that they may worship Me,” it conjures up an image of the most human of organs turned to stone, drained of all feeling for the other. Neither the suffering of his Israelite slaves nor the devastation of his Egyptian subjects by the plagues arouses in Pharaoh any trace of anguish or compassion. It is an image worthy of the 20th century, when totalitarian despots sacrificed untold millions of fellow humans on the altar of ideology!
The motif occurs no less than twenty times in the course of the exodus story. Though concrete and graphic, it does pose a theological problem. Half the references attribute the loss of feeling to divine intent: God appears to strip Pharaoh of the capacity to respond affirmatively. At the beginning of ourparasha, we (and Moses) are forewarned: “But I will harden Pharaoh’s heart, that I may multiply My signs and marvels in the land of Egypt (Exodus 7:3).” The intrusion runs roughshod over the basic principle of free will, on which the entire superstructure of Jewish law rests. Without the ability to make the good prevail in us or to control our passions, we can hardly be held accountable by God for our actions.
It is this contradiction that the midrash tried to soften, if not resolve, on the verse: “Go to Pharaoh. For I have hardened his heart and the hearts of his courtiers, in order that I may display these My signs among them… (Exodus 10:1).” The midrash is based on the striking fact that in the narrative of the first five plagues, not God but Pharaoh hardens his heart. In each case the biblical text leaves God out of the picture (Exodus 7:13, 22; 8:11, 15, 28; 9:7). Only when we come to the sixth plague, that of boils, does the Torah declare God to be the cause of Pharaoh’s hardheartedness: “But the Lord stiffened the heart of Pharaoh, and he would not heed them, just as the Lord had told Moses (Exodus 9:12).” Thereafter, except for the seventh plague (Exodus 9:34-35), God assumes direct responsibility for Pharaoh’s callousness (Exodus 10:1, 20, 27; 11:10; 14:8).
It is a pattern that only the most attentive of readers would ever detect. The midrash makes of it a cautionary note on the subject of free will. We are not quite as unrestricted as we would like to believe. The consequences of earlier deeds and habits continue to bedevil us. That is why Pharaoh lost the capacity to do teshuva, to make a mid-course correction in his life.
According to the midrash, God will warn a person gone astray several times. If the response is persistently obtuse and inflexible, God will eventually desist and close the door to teshuva in order to exact punishment for the sins committed.
This is what happened in the case of the wicked Pharaoh. God sent five warnings to which he paid no attention. Finally, God said to him: `You stiffened your neck and hardened your heart. I now will render you still more vile.’ Hence the text read: `For I have hardened his heart.’
How ironic that Pharaoh becomes a lesson on the limits of repentance! His sustained assault against Joseph’s people had inured him to the cries of the afflicted. Egyptian fear of Israel’s numbers may have prompted the original decision to enslave them, but it was the lavish profits born of that slave labor which made the decision irreversible. And greed, not fear, drove Pharaoh to his death in a vain effort to recover the slaves he had freed unwillingly. “What is this we have done, releasing Israel from our service (Exodus 14:4)?”
Clearly, the motivation of Pharaoh’s oppressive policy had shifted from self-protection to self-enrichment, and as it did it became ever less susceptible to alteration. Would America’s South have ever voluntarily ended its economic dependence on a form of labor grown morally repulsive? Neither leadership had the capacity any longer of radical change without great violence. The Civil War was as necessary and inevitable as the Ten Plagues.
This is the deeper meaning of our midrash. Judaism’s belief in the human capacity to change, to do teshuva, stands in marked contrast to the notion that surfaces often in the history of Christianity, that our fate is wholly predetermined by God, a function of God’s free and sovereign grace. For all God’s grandeur, Judaism never denied humanity its own space, in which its destiny, individually and collectively, is shaped exclusively by human hands.
And yet, reality moderates our faith. Experience testifies that human freedom is never absolute. A person unloved as a child will have great difficulty loving as an adult. Behavior patterns are no less addictive than drugs. Judaism takes the middle ground. With enough faith, support and determination most habits can be broken, most paths rerouted, most relationship restored. The primary function of Judaism is to alert us before it is too late. This is the reason for an annual season of introspection that offers us a reprieve but not a clean slate. At its peak moments of intensity (during the recitation of the u-netane tokef) we declaim mutedly that “Penitence, prayer and good deeds can annul the severity of the decree.”

Greta Hort Plague Sequence Theory as recounted in Bible and Spade, Spring 2004
...According to Hort's theory, each Plague recounted in the book of Exodus occurs in the correct 'natural' sequence, all triggered by one underlying cause - a single and extraordinarily severe occurrence of the annual Nile flood in July. This torrent washed down a massive load of 'red' mud along with the main reddening agent, the red algae, which discolored and contaminated the water in a red tide (or an HAB, as biologists now prefer to call it). The supposedly red mud-algae together created the Plague of Blood. Hort names two species of algae as the culprits. Then followed the deadly chain reaction of successive Plagues, says Hort.

The algae killed the fish. Dead fish developed anthrax, sickening the frogs and driving them ashore. Dead frogs contaminated the soil, infecting and killing livestock animals with internal anthrax. Biting flies proliferated in the decaying plants left by the heavy flooding then carried the especially virulent skin anthrax from dead cattle to the live cattle and humans, causing the plague of 'boils.' The severe weather that had caused the unusual torrent then brought hail and locusts. The excessive load of mud from the high Nile flood dried into an unusually heavy covering of powdered silt. A violent dust storm lofted the silt into the air, creating an extraordinary darkness. The first fruits of crops were destroyed - rather than firstborn children - in the final Plague according to Hort, claiming error in translation of the Biblical account. Some of the Plagues may have missed the Israelites due to a natural sheltering effect of the Eastern Nile Delta valley they dwelled in, she claims. Hort suggests the Israelites left Egypt because they had food the Egyptians wanted and would have taken from them, so if they stayed they would 'die either by violence or starvation.'

Sarna, Nahum M.. Exploring Exodus (p. 106). Knopf Doubleday Publishing Group. Kindle Edition.
here we encounter for the first time a new development in the religion of Israel. The text unequivocally speaks of judgments upon the gods of Egypt. The Book of Genesis ignores the theme of the struggle against paganism. The Patriarchal narratives exhibit no tension between the religion of the founding fathers and that of the nations with whom they come in contact. Only oblique hints of difference are occasionally perceptible, such as when Jacob swears “by the Fear of his father Isaac” while Laban, the Aramean, takes an oath by his ancestral deities;47 or when Jacob demands that the members of his household rid themselves of the “alien gods” in their midst.48 The notion of a war on polytheism, however, is first found here in our Exodus narrative, and it becomes henceforth one of the Bible’s major themes. Apart from the explicit statement of Exodus 12:12 and Numbers 33:4, and the nature of some of the plagues, it also finds subtle expression in other ways in the plagues narrative. The inability of the magicians to reproduce the plagues after a while,49 the transcendent ability of God to manipulate nature in order to realize His purposes, and the protection from the effects of the plagues that He bestows upon Israel—all are calculated to undermine faith in what the Egyptians accepted as divinities. Without doubt, the Israelite war on polytheism begins with the advent of Moses as liberator from Egyptian bondage.50 Its ultimate formulation will appear in the Ten Commandments: “You shall have no other gods besides Me.”