The occasion on which the plagues were sent is described in Exod 3-12.

1. The plague of blood.When Moses and Aaron came before Pharaoh, a miracle was required of them. Then Aaron's rod became "a serpent (Authorized Version), or rather "a crocodile." Its being changed into an animal reverenced by all the Egyptians, or by some of them, would have been an especial warning to Pharaoh, The Egyptian magicians called by the king produced what seemed to be the same wonder, yet Aaron's rod swallowed up the others. (Exodus 7:3-12) This passage, taken alone would appear to indicate that the magicians succeeded in working wonders, but, if it is compared with the others which relate their opposition on the occasions of the first three plagues, a contrary inference seems more reasonable for the very first time that Moses wrought his miracle without giving previous notice, the magicians "did so with their enchantments," but failed. A comparison with other passages strengthens us in the inference that the magicians succeeded merely by juggling. After this warning to Pharaoh, Aaron, at the word of Moses, waved his rod over the Nile, and the river was turned into blood, with all its canals and reservoirs, and every vessel of water drawn from them; the fish died, and the river stank. The Egyptians could not drink of it, and digged around it for water. This plague was doubly humiliating to the religion of the country, as the Nile was held sacred, as well as some kinds of its fish, not to speak of the crocodiles, which probably were destroyed. (Exodus 7:16-25) Those who have endeavored to explain this plague by natural causes have referred to the changes of color to which the Nile is subject, the appearance of the Red Sea, and the so called rain and dew of blood of the middle ages; the last two occasioned by small fungi of very rapid growth. But such theories do not explain why the wonder happened at a time of year when the Nile is most clear nor why it killed the fish and made the water unfit to he drunk.
2. The plague of frogs . --When seven days had passed after the first plague, the river and all the open waters of Egypt brought forth countless frogs, which not only covered the land but filled the houses, even in their driest parts and vessels, for the ovens and kneading-troughs are specified. This must have been an especially trying judgment to the Egyptians, as frogs were included among the sacred animals. (Exodus 8:1-15)
3. The plague of lice . --The dry land was now smitten by the rod, and very dust seemed turned into minute noxious insects, so thickly did they swarm on man and beast, or rather "in" them. The scrupulous cleanliness of the Egyptians would add intolerably to the bodily distress of this plague, by which also they again incurred religious defilement. As to the species of the vermin, there seems no reason to disturb the authorized translation of the word. The magicians, who had imitated by their enchantments the two previous miracles, were now foiled. They struck the ground, as Aaron did, and repeated their own incantations. but it was without effect. (Exodus 8:16-19)
4. The plague of flies . --After the river and the land, the air was smitten, being filled with winged insects, which swarmed in the houses and devoured the land, but Goshen was exempted from the plague. The word translated "swarms of flies" most probably denotes the great Egyptian beetle, Scarabaeus sacer , which is constantly represented in their sculptures. Besides the annoying and destructive habits of its tribe, it was an object of worship, and thus the Egyptians were again scourged by their own superstitions. (Exodus 8:20-32)
5. The plague of the murrain of beasts . --Still coming closer and closer to the Egyptians, God sent a disease upon the cattle, which were not only their property but their deities. At the precise time of which Moses forewarned Pharaoh, all the cattle of the Egyptians were smitten with a murrain and died, but not one of the cattle of the Israelites suffered. (Exodus 9:1-7)
6. The plague of boils --From the cattle the hand of God was extended to the persons of the Egyptians. Moses and Aaron were commanded to take ashes of the furnace, and to "sprinkle it toward the heaven in the sight of Pharaoh." It was to become "small dust" throughout Egypt, and "be a boil breaking forth [with] blains upon man and upon beast." (Exodus 9:8-12) This accordingly came to pass. The plague seems to have been the leprosy, a fearful kind of elephantiasis which was long remembered as "the botch of Egypt." (28:27,35)
7. The plague of hail . --The account of the seventh plague is preceded by a warning which Moses was commanded to deliver to Pharaoh, respecting the terrible nature of the plagues that were to ensue if he remained obstinate. Man and beast were smitten, and the herbs and every tree broken, save in the land of Goshen. The ruin caused by the hail was evidently far greater than that effected by any of the earlier plagues. Hail is now extremely rare, but not unknown, in Egypt, and it is interesting that the narrative seems to imply that if sometimes falls there. (Exodus 9:13-34)
8. The plague of locusts . --The severity of this plague can be well understood by those who have been in Egypt in a part of the country where a flight of locusts has alighted. In this case the plague was greater than an ordinary visitation, since it extended over a far wider space, rather than because it was more intense; for it is impossible to imagine any more complete destruction than that always caused by a swarm of locusts. (Exodus 10:1-20)
9. The plague of darkness . --"There was a darkness in all the land of Egypt three days;" while "all the children of Israel had light in their dwellings." It has been illustrated by reference to the samoom and the hot wind of the Khamaseen. The former is a sand-storm which occurs in the desert, seldom lasting more than a quarter of an hour or twenty minutes, but for the time often causing the darkness of twilight, and affecting man and beast. The hot wind of the Khamaseen usually blows for three days and nights, and carries so much sand with it that it produces the appearance of a yellow fog. It thus resembles the samoom, though far less powerful and less distressing in its effects. It is not known to cause actual darkness. The plague may have been an extremely severe sandstorm, miraculous in its violence and duration, for the length of three days does not make it natural since the severe storms are always very brief. (Exodus 10:21-29)
10. The death of the first-born . --Before the tenth plague Moses went to warn Pharaoh: "Thus saith the Lord, about midnight will I go out into the midst of Egypt; and all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne even to the first-born of the maidservant that is behind the mill; and all the first-born of beasts." (Exodus 11:4,5) The clearly miraculous nature of this plague, its falling upon man and in its beast; and the singling out of the firstborn, puts it wholly beyond comparison with any natural pestilence, even the severest recorded in history, whether of the peculiar Egyptian plague or of other like epidemics. The history of the ten plagues strictly ends with the death of the first-born. The gradual increase in severity of the plagues is perhaps the best key to their meaning. They seem to have been sent as warnings to the oppressor, to afford him a means of seeing God's will and an opportunity of repenting before Egypt was ruined. The lesson that Pharaoh's career teaches us seems to be that there are men whom the meet signal judgments do not affect so as to cause any lasting repentance. The following characteristics of the plagues may be specially noticed: (1) Their relation to natural phenomena. Each of the inflictions has a demonstrable connection with Egyptian customs and phenomena; each is directly aimed at some Egyptian superstition all are marvellous, not for the most part as reversing, but as developing, forces inherent in nature, and directing them to a special end. --Canon Cook . (2) Their order. They are divided first into nine and one the last one standing clearly apart from all the others. The nine are arranged in threes. In the first of each three the warning is given to Pharaoh in the morning. In the first and second of each three the plague is announced beforehand in the third, not. At the third the magicians acknowledge the finger of God; at the sixth they cannot stand before Moses; and at the ninth Pharaoh refuses to see the face of Moses any more. The gradation of the severity of these strokes is no less obvious. In the first three no distinction is made among the inhabitants of the land; in the remaining seven a distinction is made between the Israelites, who are shielded from, and the Egyptians who are exposed to, the stroke. -Kurlz, (3) Their duration. It is probable that the plagues extended through a period of several months. The first plague occurred probably during the annual inundation of the Nile, hence about the middle of June (Edersheim). The second, that of the frogs, in September, the time when Egypt often suffers in this way. The seventh (hail) came when the barley was in ear, and before the wheat was grown, and hence in February; and the tenth came in the following March or April. (4) Their significance. The first plague was directed against the Nile one of the Egyptian deities, adored as a source of life, not only to the produce of the land, but to its inhabitants. The second plague, that of the frogs, struck also at the idolatry of Egypt; for the frog was an object of worship. The third plague turned the land, which was worshipped, into a source of torment the dust produced a curse. The fourth plague consisted in the torment of either flies of a ravenous disposition, or beetles. If the former, then the air, which was worshipped, was turned into a source of exquisite annoyance; if the latter then the beetle, one of the most common of the Egyptian idols, swarmed with voracious appetite, attacking even man, as the Egyptian beetle still does and inflicting painful wounds. The fifth plague, that of murrain, struck at the cattle-worship for which Egypt was celebrated. The sixth plague, produced by the ashes scattered toward heaven in conformity with an ancient Egyptian rite, as if an invocation of the sun-god, continued the warfare of Jehovah upon Egyptian idolatry; the religious ceremony which was employed to invoke blessing brought disease. The seventh plague, beginning a new series, seems to have been aimed like those which followed, to demonstrate the power of Jehovah over all the elements, and even life itself, in contrast with the impotence of the idols. The storm and the hail came at his bidding. The locusts appeared and departed at his word. The sun itself was veiled at his command. Nay, the angel of death was held and loosed by his hand alone. The tenth plague had an immediate relation to idolatry, since it destroyed not only the first-born of man, but the first-born of beast; so that the sacred animals in the temples were touched by a power higher than those they were supposed to represent. The victory was complete; upon all the gods of Egypt, Jehovah had executed judgment. --Rev. Franklin Johnson .