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Original sin is the fallen state, or condition, of man's nature owing to an absence of sanctifying grace in his soul at conception. It was caused by Adam's first personal sin, which involved pride in wanting to be like God and disobedience in obeying his wife instead of God, as revealed in Genesis 3:6. Adam, who was created in the state of original justice, immediately lost sanctifying grace by his first sin. All people, being his bodily descendants, are consequently conceived without God's sanctifying presence in their souls.
The Catechism of the Catholic Church (Catechism) in paragraph 404 affirms that "original sin is ... a state and not an act." As the state of mortal sin is caused by an act of mortal sin, so too was the state of original sin caused by Adam's first, or original, sin. As mortal sin is the lack of sanctifying grace after being justified by baptism, similarly, original sin is the state of being without sanctifying grace before baptism. And, as the sacrament of confession restores God's sanctifying presence to the soul, so too does baptism remit original sin by introducing God's justifying presence into the soul.
In Adam's state of original justice, he enjoyed what's called the preternatural gifts, which included infused science, as shown in Genesis 2:19-20 by his ability to name the animals; the gift of integrity allowing his passions to freely follow reason; and the absence of bodily suffering and death.
By his fall Adam lost sanctifying grace and the preternatural gifts. He acquired what's called concupiscence — defects that include a darkened intellect, making learning arduous; a will tainted with malice, inclining it to choose sin; and unruly passions ever ready to rebel against reason.
Paragraph 405 of the Catechism affirms that Adam's lineage retains, even after Baptism, what's called the stain of original sin, which is concupiscience. Fallen human nature, even when in a state of grace, is thus still "subject to ignorance, suffering and the dominion of death, and inclined to sin."
Take a look into the fall of Adam and Eve in season one of Church Militant's Premium show Armor of God—The Creation and Fall of Man.