Just what does Infant Baptism accomplish as regards Original Sin? The answer to this question indicates that the human soul is created by God, and without any lack of saving grace, at the precise instant that the soul is infused into the newly propagated human body.
The sin and death we inherit from Adam, says Aquinas, are passed in the physical human nature by the human father.  God does not, the answer declares, create a deprived human soul but the newly created human soul is 'infected' instantly upon infusion with that body.  At that moment the human soul becomes devoid of saving grace and stands in need of salvation.
Infant Baptism is intended to remediate this 'original sin' in the Catholic Church:

1250 Born with a fallen human nature and tainted by original sin, children also have need of the new birth in Baptism to be freed from the power of darkness and brought into the realm of the freedom of the children of God, to which all men are called [Cf Col 1:12–14]. [Catechism of the Catholic Church]

My assumption is that infant baptism does not remove original sin from the physical aspect of humanity, otherwise no further sacraments would be necessary.  1 John warns us not to say that we have no sin:

If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say we have not sinned, we make him a liar, and his word is not in us. - 1 John 1:8-10 

And Paul details his current struggle with the sin that dwells in his physical members even after his experience on the Damascus Road and his Baptism and call to Apostleship:

For we know that the law is spiritual, but I am of the flesh, sold under sin. For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. Now if I do what I do not want, I agree with the law, that it is good. So now it is no longer I who do it, but sin that dwells within me. For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out. For I do not do the good I want, but the evil I do not want is what I keep on doing. Now if I do what I do not want, it is no longer I who do it, but sin that dwells within me. So I find it to be a law that when I want to do right, evil lies close at hand. For I delight in the law of God, in my inner being, but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members. Wretched man that I am! Who will deliver me from this body of death? - Romans 7:14-24

Additionally, Paul looks to a future day (the return of the Lord Jesus and the resurrection) when he will be delivered from his body of death so it is clear that he is not currently experiencing freedom from the presence of sin in his members.
Infant Baptism then must somehow remove the stain of sin that the human soul derives from it's infusion into the body and return it to it's intended created state possessed of sanctifying grace.
If this is the case, what keeps immediate re-infection of the soul from happening since the soul/body union still remains and the body is still tainted and infectious?
The Catechism of the Council of Trent (Roman Catechism) lists 5 effects of the sacrament of baptism:

Remission of sin
Remission of all punishment due to sin
Grace of regeneration
Infused virtues and incorporation with Christ
Character of Christian


&quot;what keeps immediate re-infection of the soul from happening&quot; after baptism?

Concupiscence remains after baptism, but it is not sin (cf. the § &quot;Concupiscence Which Remains After Baptism Is No Sin&quot; where the Catechism describes the first effect of baptism).
Council of Trent, session 5:


If any one denies, that, by the grace of our Lord Jesus Christ, which is conferred in baptism, the guilt of original sin is remitted […] let him be anathema.[…]this holy synod confesses and is sensible, that in the baptized there remains concupiscence, or an incentive (to sin); which, whereas it is left for our exercise, can not injure those who consent not, but resist manfully by the grace of Jesus Christ; yea, he who shall have striven lawfully shall be crowned. (2 Tim. ii. 5). This concupiscence, which the apostle sometimes calls sin (Rom. vi. 12; vii. 8), the holy Synod declares that the Catholic Church has never understood it to be called sin, as being truly and properly sin in those born again, but because it is of sin, and inclines to sin. And if any one is of a contrary sentiment, let him be anathema.