Objection 1. It would seem that there are many original sins in one man. For it is written (Psalm 1:7): "Behold I was conceived in iniquities, and in sins did my mother conceive me." But the sin in which a man is conceived is original sin. Therefore there are several original sins in man.
Objection 2. Further, one and the same habit does not incline its subject to contraries: since the inclination of habit is like that of nature which tends to one thing. Now original sin, even in one man, inclines to various and contrary sins. Therefore original sin is not one habit; but several.
Objection 3. Further, original sin infects every part of the soul. Now the different parts of the soul are different subjects of sin, as shown above (Article 74). Since then one sin cannot be in different subjects, it seems that original sin is not one but several.
On the contrary, It is written (John 1:29): "Behold the Lamb of God, behold Him Who taketh away the sin of the world": and the reason for the employment of the singular is that the "sin of the world" is original sin, as a gloss expounds this passage.
I answer that, In one man there is one original sin. Two reasons may be assigned for this. The first is on the part of the cause of original sin. For it has been stated (81, 2), that the first sin alone of our first parent was transmitted to his posterity. Wherefore in one man original sin is one in number; and in all men, it is one in proportion, i.e. in relation to its first principle. The second reason may be taken from the very essence of original sin. Because in every inordinate disposition, unity of species depends on the cause, while the unity of number is derived from the subject. For example, take bodily sickness: various species of sickness proceed from different causes, e.g. from excessive heat or cold, or from a lesion in the lung or liver; while one specific sickness in one man will be one in number. Now the cause of this corrupt disposition that is called original sin, is one only, viz. the privation of original justice, removing the subjection of man's mind to God. Consequently original sin is specifically one, and, in one man, can be only one in number; while, in different men, it is one in species and in proportion, but is numerically many.
Reply to Objection 1. The employment of the plural--"in sins"--may be explained by the custom of the Divine Scriptures in the frequent use of the plural for the singular, e.g. "They are dead that sought the life of the child"; or by the fact that all actual sins virtually pre-exist in original sin, as in a principle so that it is virtually many; or by the fact of there being many deformities in the sin of our first parent, viz. pride, disobedience, gluttony, and so forth; or by several parts of the soul being infected by original sin.
Reply to Objection 2. Of itself and directly, i.e. by its own form, one habit cannot incline its subject to contraries. But there is no reason why it should not do so, indirectly and accidentally, i.e. by the removal of an obstacle: thus, when the harmony of a mixed body is destroyed, the elements have contrary local tendencies. In like manner, when the harmony of original justice is destroyed, the various powers of the soul have various opposite tendencies.
Reply to Objection 3. Original sin infects the different parts of the soul, in so far as they are the parts of one whole; even as original justice held all the soul's parts together in one. Consequently there is but one original sin: just as there is but one fever in one man, although the various parts of the body are affected.
Source: Summa Theologica (New Advent)