And Moses made a serpent of brass (As the Lord commanded), 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.(Num 21:9 KJV)
And as Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up; that whoever believes may in Him have eternal life. (John 3:14-15 NASB)
The important thing here is that before looking up to the Serpent of Brass there must first be a "bite" from a serpent. Before any healing can happen, there must first be sickness, before any binding of wounds, there must be brokenness, before salvation and restoration there must be a falling short of glory of God.
Negative first before the Positive,the natural before that which is spiritual, the carnal Law before the spiritual Law.
"Let there be Light" God says "and there was light" but, there was darkness first. The emphasis from that verse was not the biting of the serpent, neither was it on the people that were bitten rather on the Serpent of Brass that was hung on the tree. That's the focus, the objects of all worship and adoration.
Serpent that was biting the people here means…sins, craftiness, lies, evil works, the devil himself…the antidotes to these things is nothing but looking up to Serpent of Brass for healing.