And the people spake against God, and against Moses, Wherefore have ye brought us up out of Egypt to die in the wilderness? for there is no bread, neither is there any water; and our soul loatheth this light bread.
 And the LORD sent fiery serpents among the people, and they bit the people; and much people of Israel died.
 Therefore the people came to Moses, and said, We have sinned, for we have spoken against the LORD, and against thee; pray unto the LORD, that he take away the serpents from us. And Moses prayed for the people.
 And the LORD said unto Moses, Make thee a fiery serpent, and set it upon a pole: and it shall come to pass, that every one that is bitten, when he looketh upon it, shall live.
 And Moses made a serpent of brass, and put it upon a pole, and it came to pass, that if a serpent had bitten any man, when he beheld the serpent of brass, he lived.
 And the children of Israel set forward, and pitched in Oboth.
The pole on which Blake pictures the fiery serpent erected by Moses is in the
Moses & the Brazen Serpent
The image of the serpent wrapped around a pole became the symbol of healing. Christ to whom we look for healing was raised upon the pole of the cross himself.
Although Blake considered Moses to be an inspired prophet he disagreed with Moses' accounts of God as vengeful. That God would send the fiery serpents on the people of Israel because they complained of their hardships, does not agree with the picture of the loving, forgiving God of the New Testament. A benevolent God could lead the people into finding a way to deal with a disease they encountered without having sent the disease as punishment.
Is this passage from Blake we find the right/left symbolism he used, and 'Satan wound round by the Serpent & falling headlong', in association with Moses and the Tables of Stone.
DESCRIPTIONS OF THE LAST JUDGMENT, (E 552)
" To Ozias Humphry Esqre
The right hand of the Design is appropriated to the
Resurrection of the Just the left hand of the Design is
appropriated to the Resurrection & Fall of the Wicked
Immediately before the Throne of Christ is Adam & Eve
kneeling in humiliation
Race Abraham & Moses kneel on each side beneath them from the
cloud on which Eve kneels [ & beneath Moses & from the Tables
of Stone which utter lightnings] is seen Satan wound round
by the Serpent & falling headlong the Pharisees appear on the
left hand pleading their own righteousness before the Throne of
The fiery serpent in contemporary news is available in these articles: CNN, Carter Center, Dr Bob.