All creation was affected by the fall, flesh included.
If this were true then mankind could drive the bad genes out by genetic breeding - you know, like Hitler tried to do with his with his Übermensch. As I've said elsewhere, if God made us evil then the fault and guilt of our sinful ways is God's. Let me suggest another way which doesn't include Gnostic heresies.
“Justice is uprightness rectitude-of-will kept for its own sake.” [St. Anselm, On Truth, 12]. Continuing Anselm said, “Justice is not rightness of knowledge or rightness of action but is rightness of will.” [St. Anselm, On Truth, 12]. The will is contained in the intellect of man, thus ‘to will’ something is, by the definition, an act. As we are, Adam was free to act either in obedience or in disobedience. Prior to his original act of rebellion, Adam 'abides' in God. This abiding would have been more intense than abiding in Christ when partaking in the Eucharist. [Cf. John 6:57].
Adam's justice was part of his creation. Adam's graces were imparted in his creation and inexplicably joined the cardinal virtues having rights to honorable prudence, temperance, and fortitude in all moral acts. These gifts were inheritable to his progeny, had Adam not sinned we would have enjoyed the same honors given to Adam.
Because of the fist sin we bear both guilt and punishment as a people because of this original sin. We inherit the punishment and guilt in our birth being the progeny of Adam. Prior to his original to Adam’s rebellion, it could be said Adam 'abided' in God as we are invited to abide in Christ in the Eucharist after Baptism [Cf. John 6:57]. The punishment of original sin is not something put into us, or something that re-creates us into sin, rather it is the withdrawal of God's graces of original justice that once belonged to the patriarch of all men. Justice then was a grace we would have rightly inherited as his progeny had it not been Adam’s sin. Prior to the fall, Adam stood before God as a just man. The original man was created with a soul that was perfectly joined to the intellect and perfectly united with the will of God. The deprivation of justice finds its origin in Adam’s sin through his act of revolutionary disobedience; it is our heritage.
Without justification found in Baptism all acts, whether good or evil, are avaricious and thus not charitable as any good is done in expectation of gain. Hence, without Baptismal justification one cannot be saved. Justice then is a moral quality or habit, when perfected joins our will to an enlightened understanding to the will of God. When perfected we "walk with God".
So, we see that sin or evil is not in the flesh as we are creatures of God. Rather the guilt and punishment of the original sin of Adam is passed on to us not by something added to man but something man was deprived of, justice. “Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God.”
In Baptism we are 'made right' with God, i.e. righteous through the efficacious grace. Man is 'reborn' into His Kingdom marked as belonging to Christ. Being made right with God a reborn man can, if he will, become justified to the will of God. Hence in Baptism we see the forgiveness of sins making us 'right' with God, beginning our life's journey of being 'justified'. Justification then is a state of being going from unjust prior to Baptism to a state of perfectly justified. We remain part of the corporate army of Satin until reborn in baptism into the Kingdom of where a semblance of justification is restored in the founts of the Church. In the Kingdom, we strive perfection "as also your heavenly Father is perfect" [Matthew 5:48].
Nor can justification be mere mental assent, you can't come to know what you don't love and the unrighteous do not love God. We perfect justification through charity and the love of God with the whole of the heart, soul and mind [Matthew 22:37].
JoeT