We are not born pure. We are all stained by original sin from our conception. It is the lack of the necessary state of grace to live the supernatural life of God and be with him in heaven. It is not a personal guilt, as if we were guilty of the sin of Adam. It can be thought more of man being merely man, possessing only his natural life and powers, disordered toward selfishness and sin.
We overcome it through only one means: Baptism. Ordinarily, Baptism is through water and the Trinitarian batismal formula, although there are two other ways wherein the unbaptized also receive the grace of baptism: through desire and through blood.
Through Baptism, the guilt of original sin is washed away and we gain sanctifying grace–the supernatural life of God, the grace of adoption. Only with sanctifying grace can one live with God in heaven. It is why the Catholic Church insists that infants be baptized as soon as possible.